usb: add support to the generic PHY framework in OTG

This patch adds support of the PHY framework in OTG and keeps the USB
PHY compatibility. Here the only modification is to add PHY member in
the OTG structure, along with the USB PHY one.

Signed-off-by: Antoine Tenart <antoine.tenart@free-electrons.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>