ata: ahci: Add support for AHCI compliant MSM host controller

Add a glue driver for AHCI platform driver to support SATA
controller on MSM platforms. The SATA controller on MSM adheres
to AHCI 1.31 specification.

The glue driver acts as a device driver for compatible msm ahci device
and registers a AHCI platform device in its probe, AHCI platform driver
then acts as a device driver for created AHCI device. All the necessary
msm platform specific initialization is handled by msm_ahci driver while
keeping ahci_platform generic enough.

Change-Id: Id435116ed8297629df6f18d7ebb5ec80b84ed384
Signed-off-by: Venkat Gopalakrishnan <venkatg@codeaurora.org>
Signed-off-by: Sujit Reddy Thumma <sthumma@codeaurora.org>