arm: mvebu: Add RTC support for Armada 370 and Armada XP

The Armada 370 and Armada XP Socs have the same controller that the
one used in the orion platforms. This patch updates the device tree
for these SoCs.

Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Acked-by: Andrew Lunn <andrew@lunn.ch>
Tested-by: Florian Fainelli <florian@openwrt.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>