dt-bindings: sun6i-a31-mipi-dphy: Add optional direction property

dt-bindings: sun6i-a31-mipi-dphy: Add optional direction property

The Allwinner A31 MIPI D-PHY block supports both tx and rx directions,
although each instance of the block is meant to be used in one
direction only. There will typically be one instance for MIPI DSI and
one for MIPI CSI-2 (it seems unlikely to ever see a shared instance).

Describe the direction with a new allwinner,direction property.
For backwards compatibility, the property is optional and tx mode
should be assumed by default.

Signed-off-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220415152138.635525-2-paul.kocialkowski@bootlin.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>