media: cadence: csi2rx: Add external DPHY support

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>