Platform/Sgi: Add CPU container for RD-N1-Edge

The RD-N1-Edge platform includes two clusters with four single-thread
CPUs. Add processor container devices for the two clusters on the
RD-N1-Edge platform and move the existing processor devices into
respective processor containers.

Signed-off-by: Pranav Madhu <pranav.madhu@arm.com>
Reviewed-by: Thomas Abraham <thomas.abraham@arm.com>
Reviewed-by: Sami Mujawar <sami.mujawar@arm.com>