scsi: ufs: add trace support for ufs power management

Add ftrace support for profiling ufs power management latencies.
Usage:
	echo 1 > /sys/kernel/debug/tracing/events/ufs/enable
	cat /sys/kernel/debug/tracing/trace_pipe

Change-Id: If5c063179cc69a500d4902eb00b4ca3dc16e27f0
Signed-off-by: Sujit Reddy Thumma <sthumma@codeaurora.org>