Platform/Sgi: Add support to disable isolated cpus

Isolated CPUs are those that are not to be used on the platform for
various reasons. The isolated CPU list is an array of MPID values of
the CPUs that have to be isolated. This list is supplied via the
NT_FW_CONFIG dtb.

Add support to search for isolated CPUs MPID list and, if present,
update the MADT table to disable the corresponding CPUs.

Signed-off-by: Nishant Sharma <nishant.sharma@arm.com>
Change-Id: I9f7af896c1bcf0e0845b8b4f9f2bc199f15e6cc2