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>