drm/bridge: cdns: Convert to phy framework

Now that we have everything we need in the phy framework to allow to tune
the phy parameters, let's convert the Cadence DSI bridge to that API
instead of creating a ad-hoc driver for its phy.

Acked-by: Sean Paul <sean@poorly.run>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/8772ad061f07cd91fa48bb05880095b25eccec08.1548085432.git-series.maxime.ripard@bootlin.com