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

arch/arm/boot/dts/armada-xp-linksys-mamba.dts

@@ -95,6 +95,11 @@
 
 		internal-regs {
 
+			rtc@10300 {
+				/* No crystal connected to the internal RTC */
+				status = "disabled";
+			};
+
 			/* J10: VCC, NC, RX, NC, TX, GND  */
 			serial@12000 {
 				status = "okay";