usb: phy: add a new driver for omap usb2 phy

All phy related programming like enabling/disabling the clocks, powering
on/off the phy is taken care of by this driver. It is also used for OTG
related functionality like srp.

This also includes device tree support for usb2 phy driver and
the documentation with device tree binding information is updated.

Currently writing to control module register is taken care in this
driver which will be removed once the control module driver is in place.

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