dt-bindings: Add compatible for BCM2711 DSI1

DSI1 on BCM2711 doesn't require the DMA workaround that is used
on BCM2835/6/7, therefore it needs a new compatible string.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20201203132543.861591-7-maxime@cerno.tech