drm/vc4: Add DPI driver

The DPI interface involves taking a ton of our GPIOs to be used as
outputs, and routing display signals over them in parallel.

v2: Use display_info.bus_formats[] to replace our custom DT
    properties.
v3: Rebase on V3D documentation changes.
v4: Fix rebase detritus from V3D documentation changes.

Signed-off-by: Eric Anholt <eric@anholt.net>
Acked-by: Rob Herring <robh@kernel.org>