ARM: dts: Move BCM2711 RPi specific into separate dtsi

There is a lot of Raspberry Pi specific stuff (neither SoC or board
specific) for the BCM2711 which is currently in the RPi 4 B dts. In order
to avoid copy & paste for every new BCM2711 based Raspberry Pi, move it
into a separate dtsi.

Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Link: https://lore.kernel.org/r/1622981777-5023-4-git-send-email-stefan.wahren@i2se.com
Signed-off-by: Nicolas Saenz Julienne <nsaenz@kernel.org>