rpi5: Use devicetree to retrieve board revision

Firmware on RPi5 return error on board revision query
through firmware interface, but on the other hand it fills
"linux,revision" in "system" node, so use it to detect board
revision.

	system {
		linux,revision = <0xc04170>;
		linux,serial = <0x6cf44e80 0x3c533ede>;
	};

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>