drm/vc4: Separate VEC compatible variants

drm/vc4: Separate VEC compatible variants

The VEC's DAC on BCM2711 is slightly different compared to the one on
BCM283x and needs different configuration. In particular, bit 3
(mask 0x8) switches the BCM2711 DAC input to "self-test input data",
which makes the output unusable. Separating two compatible variants in
devicetrees and the DRM driver was therefore necessary.

The configurations used for both variants have been borrowed from
Raspberry Pi (model 3B for BCM283x, 4B for BCM2711) firmware defaults.

Signed-off-by: Mateusz Kwiatkowski <kfyatek+publicgit@gmail.com>