video: sunxi: dw-hdmi: Probe driver by compatible

Currently the sunxi dw-hdmi driver is probed unconditionally,
even if there is no such device.

Switch the driver to probing via a compatible string. This brings many
benefits; the driver is only probed when needed, and now it can read the
DT node.

Signed-off-by: Jernej Skrabec <jernej.skrabec@gmail.com>
Signed-off-by: Samuel Holland <samuel@sholland.org>