usb: phy: Add support to read operational register offset from devicetree

Some of used operational registers' offset is changed on newer platforms.
Hence to use same driver across different platforms, it is needed to pass
those register offset value from devicetree. This change adds required
functionality for same.

CRs-Fixed: 2066314
Change-Id: Id44b7a6f6fc96f54f7fef54fbd38a7da2495a928
Signed-off-by: Mayank Rana <mrana@codeaurora.org>