feat(plat/arm/sgi): read isolated cpu mpid list from sds

Add support to read the list of isolated CPUs from SDS and publish this
list via the non-trusted firmware configuration file for the next stages
of boot software to use.

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.

Signed-off-by: Nishant Sharma <nishant.sharma@arm.com>
Change-Id: I4313cf025f4c9e9feffebca2d35b259f5bafce69