dt-bindings: display: vc4: Document BCM2711 VC5

The BCM2711 comes with a new VideoCore. Add a compatible for it.

Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Tested-by: Chanwoo Choi <cw00.choi@samsung.com>
Tested-by: Hoegeun Kwon <hoegeun.kwon@samsung.com>
Tested-by: Stefan Wahren <stefan.wahren@i2se.com>
Reviewed-by: Rob Herring <robh+dt@kernel.org>
Reviewed-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/1075091c50b378505a9a89f760af11f0e1e26e5b.1599120059.git-series.maxime@cerno.tech