phy: ufs: add support to control the pull down in Tx driver

Per new PHY design, the Tx driver pull down should be enabled before analog
collapse and disabled after analog power is restored. For other PHYs
without the Tx driver pull down, this change has no impact.

Change-Id: I6b01a75bc357f94b209f743d42b460c717a3ca3c
Signed-off-by: Can Guo <cang@codeaurora.org>
Signed-off-by: Sayali Lokhande <sayalil@codeaurora.org>