The driver assumes that the platform uses the device tree, and thus relies
on some fields (of_node) being declared in some structures (drm_bridge).

This isn't true for all platforms, so make sure we can only compile the
ANX6345 on platforms where DT support is selected.

Cc: Torsten Duwe <duwe@lst.de>
Fixes: 6aa19269 ("drm/bridge: Add Analogix anx6345 support")
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20191112204551.541449-1-maxime@cerno.tech