ARM: dts: armada-38x: Adjust mbus controller description on Armada 38x

The mbus binding had been extended more than two years ago, but the
device tree files for Armada 38x didn't change.

Adding this third entry will allow the mbus going to suspend which was
the last thing preventing the SoC going to standby mode
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>

arch/arm/boot/dts/armada-38x.dtsi

@@ -360,7 +360,8 @@ coreclk: mvebu-sar@18600 {
 
 			mbusc: mbus-controller@20000 {
 				compatible = "marvell,mbus-controller";
-				reg = <0x20000 0x100>, <0x20180 0x20>;
+				reg = <0x20000 0x100>, <0x20180 0x20>,
+				      <0x20250 0x8>;
 			};
 
 			mpic: interrupt-controller@20a00 {