scsi: ufs: Add sysfs node to dynamically control clock gating

Provide an option to enable/disable clock gating during runtime.
Write 1 or 0 to "clkgate_enable" sysfs node to enable/disable
clock gating.

Change-Id: I69a20e66cf91bb889ac3942eb2d20d5adc289d03
Signed-off-by: Sahitya Tummala <stummala@codeaurora.org>