usb: phy: add a new driver for usb part of control module

Added a new driver for the usb part of control module. This has an API
to power on the USB2 phy and an API to write to the mailbox depending on
whether MUSB has to act in host mode or in device mode.

Writing to control module registers for doing the above task which was
previously done in omap glue and in omap-usb2 phy will be removed.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>