rpi5: Use devicetree as alternative way to read IO base addresses

MBOX and Watchdog on RPi5/bcm2712 have a different base IO offsets.
Find them via devicetree blob passed by bootloader.

Signed-off-by: Dmitry Malkin <dmitry@bedrocksystems.com>
Reviewed-by: Matthias Brugger <mbrugger@suse.com>
Tested-by: Jens Maus <mail@jens-maus.de>
Tested-by: Darko Alavanja <darko.alavanja@konsulko.com>
Signed-off-by: Ivan T. Ivanov <iivanov@suse.de>
Signed-off-by: Matthias Brugger <mbrugger@suse.com>