usb: rename phy to usb_phy in OTG

This patch prepares the introduction of the generic PHY support in the
USB OTG common functions. The USB PHY member of the OTG structure is
renamed to 'usb_phy' and modifications are done in all drivers accessing
it. Renaming this pointer will allow to keep the compatibility for USB
PHY drivers.

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