scsi: ufs: add trace event for ufs commands

Use the ftrace infrastructure to conditionally trace ufs command events.
New trace event is created, which samples the following ufs command data:
- device name
- optional identification string
- task tag
- doorbell register
- number of transfer bytes
- interrupt status register
- request start LBA
- command opcode

Currently we only fully trace read(10) and write(10) commands.
All other commands which pass through ufshcd_send_command() will be
printed with "-1" in the lba and transfer_len fields.

Usage:
	echo 1 > /sys/kernel/debug/tracing/events/ufs/enable
	cat /sys/kernel/debug/tracing/trace_pipe

Change-Id: Id6e16a8c0b2a833c5453ed9923384f641e374a71
Signed-off-by: Lee Susman <lsusman@codeaurora.org>