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>
Cc: Russell King <linux@arm.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>