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.

Get the pixel rate and bpp from the subdev and pass them on to the DPHY
along with the number of lanes. All other settings are left to their
default values.

Signed-off-by: Pratyush Yadav <p.yadav@ti.com>
Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>