phy: cdns-dphy: Add Rx support

The Cadence DPHY can be used to receive image data over the CSI-2
protocol. Add support for Rx mode. The programming sequence differs from
the Tx mode so it is added as a separate set of hooks to isolate the two
paths.

The PHY is in Tx mode by default and it needs to be set in Rx mode by
setting the submode to PHY_MIPI_DPHY_SUBMODE_RX in the set_mode()
callback.

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