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

TCSR_QUSB2PHY_CX2PX_LVL_SHIFT_KEEPER register is required to updated
to clamp level shifter with QUSB PHY. On some platforms, this register
is secured. Hence add functionality to perform secure write to this
register using scm call.

Change-Id: I7bc9d0cc54057eb95f533025b9f5dc50a6841e3b
Signed-off-by: Mayank Rana <mrana@codeaurora.org>