dt: Add drm_fbN_vc4 overrides for Pi0-4

Follows up '61b138ad

 ("dt: Add overrides for drm framebuffer
allocations on Pi5")' with an equivalent for Pi0-4.

These will have no effect on most normal systems, but drm_fb0_vc4
will stop SPI displays jumping in and claiming /dev/fb0.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>