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>