scsi: ufs: add support to limit the gear speed

Max. supported gear (for both HS and PWM modes) is negotiated between
host and device during link startup. We would switch to this max.
supported gear during UFS initialization. But on some platforms
(especially emulation platforms), this negotiated max. gear may not
work reliably. Hence this change adds options to limit the max. gear
(for both HS and PWM mode) for both directions (TX and RX).

Change-Id: I30b88ec9ac8fd35ab5ee48e06e727739fb9cd6b5
Signed-off-by: Subhash Jadavani <subhashj@codeaurora.org>
[cang@codeaurora.org: Resolved trivial merge conflicts]
Signed-off-by: Can Guo <cang@codeaurora.org>