i2c: mv64xxx: Document the newly introduced Armada XP A0 compatible

The first variants of Armada XP SoCs (A0 stepping) have issues related
to the i2c controller which prevent to use the offload mechanism and
lead to a kernel hang during boot.

The commit introduces a new the compatible string
marvell,mv78230-a0-i2c for the i2c controller.

Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
cc: devicetree@vger.kernel.org
Cc: stable@vger.kernel.org # v3.12+: af8d1c63afcb: ARM: mvebu: Add support to get the ID and the revision of a SoC
Cc: stable@vger.kernel.org # v3.12+: 85e618a1be2b: ARM: mvebu: Add quirk for i2c for the OpenBlocks AX3-4 board
Cc: stable@vger.kernel.org # v3.12+: 6cf70ae928ba: i2c: mv64xxx: Fix bus hang on A0 version of the Armada XP SoCs
Cc: stable@vger.kernel.org # v3.12+
Fixes: 930ab3d403ae (i2c: mv64xxx: Add I2C Transaction Generator support)
Signed-off-by: Jason Cooper <jason@lakedaemon.net>