ARM: dts: Add Raspberry Pi 400 support

The Raspberry Pi 400 is like a Pi 4 B designed into a keyboard. But there
are some minor differences:

- higher CPU clock rate (1.8 GHz)
- different Wifi chip (BCM43456)
- power off is now handled via GPIO
- no ACT LED

Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Link: https://lore.kernel.org/r/1622981777-5023-7-git-send-email-stefan.wahren@i2se.com


Signed-off-by: Nicolas Saenz Julienne <nsaenz@kernel.org>