phy: Move Allwinner A31 D-PHY driver to drivers/phy/

Now that our MIPI D-PHY driver has been converted to the phy framework,
let's move it into the drivers/phy directory.

Reviewed-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/2447609da5b80f148c79b2b2a263a0e779f3e82f.1548085432.git-series.maxime.ripard@bootlin.com