scsi: ufs: separate dependency between UFS_QCOM and UFSHCD_PLATFORM

This change disconnects the dependency between the SCSI_UFSHCD_PLATFORM
and SCSI_UFS_QCOM.

It does so by adding the following changes:
1. Introducing SCSI_UFS_QCOM as a platform device. Its probe
   function registers a set of vops to its driver_data.
2. Adding an optional device tree sub-node, under SCSI_UFSHCD_PLATFORM.
   Now, the probe function of SCSI_UFSHCD_PLATFORM invokes the probe
   function of its sub-node (if it exists), by calling
   of_platform_populate(). It ensures that vops are set, and ready to
   be used, by the time the UFSHCD driver starts its initialization
   procedure.

Change-Id: Iaa80d2f6e3fd6f6526911064b3c39605aca96ccc
Signed-off-by: Yaniv Gardi <ygardi@codeaurora.org>