ARM: dts: bcm2711: Use compatible string for BCM2711 DSI1

Updates the compatible string for DSI1 on BCM2711 to
differentiate it from BCM2835.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Signed-off-by: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
Link: https://lore.kernel.org/r/20201203132543.861591-9-maxime@cerno.tech