ARM: mvebu: Add support for I2C on OpenBlocks AX3-4

The OpenBlocks AX3-4 board, based on the Armada XP SoC, has an I2C
bus. This patch enables this bus and sets the clock frequency of the
bus.

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