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>