arm64: dts: marvell: armada-37xx: add mailbox node

This adds the rWTM BIU mailbox node for communication with the secure
processor. The driver already exists in
drivers/mailbox/armada-37xx-rwtm-mailbox.c.

Signed-off-by: Marek Behún <marek.behun@nic.cz>
Cc: Gregory Clement <gregory.clement@bootlin.com>
Cc: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>