riscv: dts: microchip: Add mpfs' topology information

The mpfs has no cpu-map node, so tools like hwloc cannot correctly
parse the topology. Add the node using the existing node labels.

Reported-by: Brice Goglin <Brice.Goglin@inria.fr>
Link: https://github.com/open-mpi/hwloc/issues/536


Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Sudeep Holla <sudeep.holla@arm.com>