phy: omap-control: update dra7 and am437 usb2 Documentation bindings

The dra7-usb2 and am437-usb2 bindings have not yet been used.
Change them to be more elegant.

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