drm/sun4i: backend: Save pointer to device tree node

Save a pointer to the backend's underlying device tree node in its
data structure. This will be used later for downstream tcons to find
and match their respective upstream backends.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>