usb: phy: qusb: Use scm call to update LVL_SHIFTER register

QUSB2PHY_LVL_SHIFTER is required to be updated to clamp the
signals from the QUSB PHY. And for certain platforms
this register is secured. Hence, use the scm_call to
do a secure write to the register.

Change-Id: I161ffe857795759083cf13cfa98ec78ca97f5fdb
Signed-off-by: Sriharsha Allenki <sallenki@codeaurora.org>
Signed-off-by: Rohith Kollalsi <rkollals@codeaurora.org>