drm/sun4i: dw-hdmi: Make HDMI PHY into a platform device

On sunxi boards that use HDMI output, HDMI device probe keeps being
avoided indefinitely with these repeated messages in dmesg:

  platform 1ee0000.hdmi: probe deferral - supplier 1ef0000.hdmi-phy
    not ready

There's a fwnode_link being created with fw_devlink=on between hdmi
and hdmi-phy nodes, because both nodes have 'compatible' property set.

Fw_devlink code assumes that nodes that have compatible property
set will also have a device associated with them by some driver
eventually. This is not the case with the current sun8i-hdmi
driver.

This commit makes sun8i-hdmi-phy into a proper platform device
and fixes the display pipeline probe on sunxi boards that use HDMI.

More context: https://lkml.org/lkml/2021/5/16/203

Signed-off-by: Saravana Kannan <saravanak@google.com>
Signed-off-by: Ondrej Jirman <megous@megous.com>
Tested-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20210607085836.2827429-1-megous@megous.com