Some platforms like TI's J721E can have the CSI2RX paired with an
external DPHY. Add support to enable and configure the DPHY using the
generic PHY framework.

Signed-off-by: Pratyush Yadav <p.yadav@ti.com>
Signed-off-by: Vaishnav Achath <vaishnav.a@ti.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>