USB: dwc3-msm: Add support for SSPHY low power mode

Disabling LDOs and VDDCx for SS mode as part of suspend
without putting SSPHY into low power mode might cause
enumeration succeed in HS mode instead of SS mode. Hence
add support for putting PHY into retention before disabling
LDOs and VDDCX.

Change-Id: I336937e5b5d6b2124b45973bedd6e8470a4c7c8e
Signed-off-by: Vijayavardhan Vennapusa <vvreddy@codeaurora.org>