ARM: mvebu: armada-xp-openblocks-ax3-4: Disable internal RTC

There is no crystal connected to the internal RTC on the Open Block
AX3. So let's disable it in order to prevent the kernel probing the
driver uselessly. Eventually this patches removes the following
warning message from the boot log:
"rtc-mv d0010300.rtc: internal RTC not ticking"

Acked-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Cc: <stable@vger.kernel.org> # v3.8 +