phy: Add Cadence D-PHY support

Cadence has designed a D-PHY that can be used by the, currently in tree,
DSI bridge (DRM), CSI Transceiver and CSI Receiver (v4l2) drivers.

Only the DSI driver has an ad-hoc driver for that phy at the moment, while
the v4l2 drivers are completely missing any phy support. In order to make
that phy support available to all these drivers, without having to
duplicate that code three times, let's create a generic phy framework
driver.

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>