USB: phy-msm-usb: Register driver with compatible platform driver

Use platform driver register for hotpluggable device.
platform driver probe may not return -EPROBE_DEFER.
This is incompatible with probe deferring and hence must
not return -EPROBE_DEFER.

Move to platform driver register instead as clocks or
regulators can return -EPROBE_DEFER.

Change-Id: Ie0d20274ebc905859d2dcff934f7c692b0900345
Signed-off-by: Sujeet Kumar <ksujeet@codeaurora.org>