Platform/Sgi: Add CPU container for SGI-575

The SGI-575 platform includes two clusters with four single-thread CPUs.
Add processor container devices for the two clusters on the SGI-575
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>