ARM: mvebu: armada-xp-linksys-mamba: Disable internal RTC

The Mamba (like the OpenBlocks AX3) doesn't have a crystal
connected to the internal RTC - let's prevent the kernel from
probing it.

Signed-off-by: Imre Kaloz <kaloz@openwrt.org>
Cc: <stable@vger.kernel.org> # v4.0 +
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>