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