feat(board/rdn2): add a new 'isolated-cpu-list' property

Add a new property named 'isolated-cpu-list' to list the CPUs that are
to be isolated and not used by the platform. The data represented by
this property is formatted as below.

  strutct isolated_cpu_mpid_list {
          uint64_t count;
          uint64_t mpid_list[MAX Number of PE];
  }

Also, the property is pre-initialized to 0 to reserve space for the
property in the dtb. The data for this property is read from SDS and
updated during boot. The number of entries in this list is equal to the
maximum number of PEs present on the platform.

Signed-off-by: Nishant Sharma <nishant.sharma@arm.com>
Change-Id: I4119f899a273ccbf8259e0d711d3a25501c7ec64