arm: dts: vexpress: Move mcc node back into motherboard node

Commit d9258898 ("arm64: dts: arm: vexpress: Move fixed devices
out of bus node") moved the "mcc" DT node into the root node, because
it does not have any children using "reg" properties, so does violate
some dtc checks about "simple-bus" nodes.

However this broke the vexpress config-bus code, which walks up the
device tree to find the first node with an "arm,vexpress,site" property.
This gave the wrong result (matching the root node instead of the
motherboard node), so broke the clocks and some other devices for
VExpress boards.

Move the whole node back into its original position. This re-introduces
the dtc warning, but is conceptually the right thing to do. The dtc
warning seems to be overzealous here, there are discussions on fixing or
relaxing this check instead.

Link: https://lore.kernel.org/r/20200603162237.16319-1-andre.przywara@arm.com
Fixes: d9258898

 ("arm64: dts: vexpress: Move fixed devices out of bus node")
Reported-and-tested-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>