arm64: dts: armada-3720-turris-mox: add firmware node

[ Upstream commit 46d2f6d0c99f7f95600e633c7dc727745faaf95e ]

Add the node representing the firmware running on the secure processor.

Signed-off-by: Marek Behún <marek.behun@nic.cz>
Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>