char: xillybus: Add driver for XillyUSB (Xillybus variant for USB)

The XillyUSB driver is the USB variant for the Xillybus FPGA IP core.
Even though it presents a nearly identical API on the FPGA and host,
it's almost a complete rewrite of the driver: The framework for exchanging
data on a USB bus is fundamentally different from doing the same with a
PCIe interface, which leaves very little in common between the existing
driver and the new one for XillyUSB.

Signed-off-by: Eli Billauer <eli.billauer@gmail.com>
Link: https://lore.kernel.org/r/20210526100311.56327-3-eli.billauer@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

char: xillybus: Move class-related functions to new xillybus_class.c

This patch is a preparation for adding another related driver, XillyUSB.
In order to share some code between the existing Xillybus driver and the
one to be added, some functions are moved to xillybus_class.c

XILLYBUS_CLASS is added to Kconfig and is common to all drivers in this
group. The relation with the existing XILLYBUS symbol is "select" rather
than "depends on" XILLYBUS_CLASS, or else "make olddefconfig" will silently
turn off XILLYBUS, which is currently enabled in several distributions.

XILLYBUS_CLASS doesn't depend on anything else, hence using it with
"select" poses no risk for a broken configuration.

After the future addition of the XillyUSB module, the tree of symbols
will be as follows:

XILLYBUS_CLASS --+-- XILLYBUS --+-- XILLYBUS_PCIE
| |
| +-- XILLYBUS_OF
|
+-- XILLYUSB

XILLYBUS is for drivers based upon memory registers + DMA-based interfaces,
and it's combined with XILLYBUS_PCIE and/or XILLYBUS_OF.

XILLYUSB is for the USB variant only.

Or a more detailed, bottom-up outline:

* CONFIG_XILLYBUS_PCIE -> xillybus_pcie.c: Functions related to PCIe.
* CONFIG_XILLYBUS_OF -> xillybus_of.c: Functions related to Xillybus as a
peripheral on an FPGA / Processor combo chip.
* CONFIG_XILLYBUS -> xillybus_core.c: Functions that are common to the two
above, mainly access to the peripheral with memory-mapped registers and
DMA.
* CONFIG_XILLYUSB -> xillyusb.c: The driver for the USB variant, accesses
the peripheral through the USB framework.
* CONFIG_XILLYBUS_CLASS -> xillybus_class.c: The new module, which contains
the class and API parts that would otherwise appear both in
xillybus_core.c and xillyusb.c. Contains utility functions for the two
latter.

And since I'm at it, comments on the module names are added in the
Kconfig's help part.

The functions are exported with the non-GPL EXPORT_SYMBOL (a matter of
taste).

Signed-off-by: Eli Billauer <eli.billauer@gmail.com>
Link: https://lore.kernel.org/r/20210526100311.56327-2-eli.billauer@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

treewide: Add SPDX license identifier - Makefile/Kconfig

Add SPDX license identifiers to all Make/Kconfig files which:

- Have no license information of any form

These files fall under the project license, GPL v2 only. The resulting SPDX
license identifier is:

GPL-2.0-only

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

xillybus: Move out of staging

This driver has been functional and stable throughout the year it has spent
in the staging area. It has been patched for minor bugs, coding style issues
and improvements during this period.

This is the second submission of this move-out, after making several style
improvements, as suggested by Dan Carpenter.

Signed-off-by: Eli Billauer <eli.billauer@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>