ARM: mvebu: Add support for the RTC in OpenBlocks AX3-4

The OpenBlocks AX3-4 has a Seiko Instruments S-35390A as the RTC
controller.  This patch enables this RTC device in the OpenBlocks
AX3-4 Device Tree.

[Thomas Petazzoni: updated with other OpenBlocks changes, rephrased
commit log.]
Signed-off-by: Nobuhiro Iwamatsu <nobuhiro.iwamatsu.yj@renesas.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

arch/arm/boot/dts/armada-xp-openblocks-ax3-4.dts

@@ -111,6 +111,11 @@ i2c@d0011000 {
 		i2c@d0011100 {
 			status = "okay";
 			clock-frequency = <400000>;
+
+			s35390a: s35390a@30 {
+				compatible = "s35390a";
+				reg = <0x30>;
+			};
 		};
 	};
 };