scsi: ufs: Add clock initialization support

Add generic clock initialization support for UFSHCD platform
driver. The clock info is read from device tree using standard
clock bindings. A generic max-clock-frequency-hz property is
defined to save information on maximum operating clock frequency
the h/w supports.

Change-Id: I9f513c708896c000757bc98f6531d1dd14a48a86
Signed-off-by: Sujit Reddy Thumma <sthumma@codeaurora.org>