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