scsi: ufs: add support for logging custom events

Add events to command history with tag "custom". These custom events
logging are very useful to see the correlation between normal command
timings and other execution contexts like clock scaling, clock gating,
line reset handling etc.

Change-Id: I6fb7fed8ddcbcea34b868c82d43d2bf1da62f1eb
Signed-off-by: Can Guo <cang@codeaurora.org>