dt-bindings: phy: Move the Cadence D-PHY bindings

The Cadence D-PHY bindings was defined as part of the DSI block so far.
However, since it's now going to be a separate driver, we need to move the
binding to a file of its own.

Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>