Allwinner SoCs typically have a Mentor Graphics Inventra MUSB
dual role controller for USB OTG. This is need for verifying
gadget functions, so enable them by default.

Tested 'otg' mode with mass storage function.
Signed-off-by: NJagan Teki <jagan@amarulasolutions.com>
Signed-off-by: NMaxime Ripard <maxime.ripard@free-electrons.com>