usb: phy: msm-hsusb: Add support for PHY CSR registers

On some targets, the PHY may be controlled by additional PHY CSRs
that offer additional control apart from the QSCRATCH registers.
These handle functions such as controlling the internal reference
clock buffer and powering on/off the entire PHY. Add support for
these and write to these registers when entering retention.

Change-Id: I6b7c8d21bc8704c26259136abed826d1a9762ac6
Signed-off-by: Jack Pham <jackp@codeaurora.org>