phy: Distinguish between Rx and Tx for MIPI D-PHY with submodes

As some D-PHY controllers support both Rx and Tx mode, we need a way for
users to explicitly request one or the other. For instance, Rx mode can
be used along with MIPI CSI-2 while Tx mode can be used with MIPI DSI.

Introduce new MIPI D-PHY PHY submodes to use with PHY_MODE_MIPI_DPHY.
The default (zero value) is kept to Tx so only the rkisp1 driver, which
uses D-PHY in Rx mode, needs to be adapted.

Signed-off-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Signed-off-by: Pratyush Yadav <p.yadav@ti.com>
Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>