MFC r336079:

Add pnp info to imx6_ahci, and add a module makefile, and a manpage for it.

MFC r333073-r333074

r333073 by manu:
arm: Fix duplicate ahci DRIVER_MODULE

Name each ahci driver uniquely.
This remove the warning printed at each arm boot :
module_register: cannot register simplebus/ahci from kernel; already loaded from kernel

r333074 by manu:
arm: Fix duplicate ehci DRIVER_MODULE

Name each ehci driver uniquely.
This remove the warning printed at each arm boot :
module_register: cannot register simplebus/ehci from kernel; already loaded from kernel

MFC r324186:

Define a single instance of ahci_devclass and reference it from all the
attachment code for various SOCs and busses. Remove all the static and
should-have-been-static and named-differently instances of it.

This should eliminate the recently-grown build warnings about multiple
definitions when building arm kernels.

MFC r321938, r322015

r321938:
Fix the interface to imx_iomux_gpr_get/set(). The functions were defined
as taking a register number, and that would get multiplied by 4 to make
a register address. But the header file that consumers have to reference
this stuff publishes register addresses, not numbers. So now everything
works in terms of register addresses.

Note that the HDMI init code was writing into the wrong register before
this change. Apparently whatever it wrote to was harmless, and apparently
HDMI was working because uboot had set up the right bits.

r322015:
Add an ahci driver for imx6.

This was submitted by Rogiel Sulzbach (thank you!) but has a few last-minute
changes by me, mostly where the code interfaces to my still-utterly-deficient
imx6_ccm clocks implementation. So blame me for any mistakes.

Submitted by: Rogiel Sulzbach <rogiel@rogiel.com>
Differential Revision: https://reviews.freebsd.org/D11177