usb: phy: Remove unused register offsets

Remove unused register offset related enums as those are not used
for configuration within USB PHY driver. This helps to reduce
number of register offset value need to pass from devicetree with
USB QMP PHY device node.

Change-Id: Ia0581c3dc8210bfaac9a7318eb75e1ffa35440b6
Signed-off-by: Mayank Rana <mrana@codeaurora.org>