Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Merge Perforce changeset 219952 to head:

Make different bus attachments for Altera and Terasice
device drivers share the same devclass_t.

Sponsored by: DARPA, AFRL

Merge Perforce changeset 219940 to head:

Rework altera_avgen(4) to cleanly(ish) separate nexus bus
attachment from the driver itself. This should allow us to
plug in an fdt attachment more easily.

Sponsored by: DARPA, AFRL

Add altera_avgen(4), a generic device driver to be used by hard and soft
CPU cores on Altera FPGAs. The device driver allows memory-mapped devices
on Altera's Avalon SoC bus to be exported to userspace via device nodes.
device.hints directories dictate device name, permissible access methods,
physical address and length, and I/O alignment. Devices can be accessed
using read(2)/write(2), but also memory mapped in userspace using mmap(2).

Devices attach directly to the Nexus, as is common for embedded device
drivers; in the future something more mature might be desirable. There is
currently no facility to support directing device-originated interrupts to
userspace.

In the future, this device driver may be renamed to socgen(4), as it can
in principle also be used with other system-on-chip (SoC) busses, such as
Axi on ASICs and FPGAs. However, we have only tested it on Avalon busses
with memory-mapped ROMs, frame buffers, etc.

Sponsored by: DARPA, AFRL