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: I0ceb3fd7a0ac6b931862768448c921a76e32db24
Signed-off-by: Can Guo <cang@codeaurora.org>