arm64: dts: broadcom: Add reference to RPi CM4 IO Board

This adds a reference to the dts of the Raspberry Pi Compute Module 4
IO Board, so we don't need to maintain the content in arm64.

Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Link: https://lore.kernel.org/r/1628334401-6577-11-git-send-email-stefan.wahren@i2se.com


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