ata: ahci: MSM AHCI controller driver

The MSM AHCI controller driver enables AHCI interface
on Qualcomm SoC and plugs with Linux AHCI driver to
provide platform specific services. Add documentation
for msm ahci driver.

Change-Id: I3d508e6f9c4f15506541df4000bd9372d516c098
Signed-off-by: Sujit Reddy Thumma <sthumma@codeaurora.org>