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

TCSR_QUSB2PHY_CX2PX_LVL_SHIFT_KEEPER register is required to be 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.

CRs-Fixed: 1116118
Change-Id: Ica8c0257a06634194a7e5e1f4ecc0b62fca787cd
Signed-off-by: Devdutt Patnaik <dpatnaik@codeaurora.org>