spmd: Fix signedness comparison warning

With -Wsign-compare, compilers issue a warning in the SPMD code:
====================
services/std_svc/spmd/spmd_pm.c:35:22: error: comparison of integer
expressions of different signedness: 'int' and 'unsigned int'
[-Werror=sign-compare]
   35 |  if ((id < 0) || (id >= PLATFORM_CORE_COUNT)) {
      |                      ^~
cc1: all warnings being treated as errors
====================

Since we just established that "id" is positive, we can safely cast it
to an unsigned type to make the comparison have matching types.

Change-Id: I6ef24804c88136d7e3f15de008e4fea854f10ffe
Signed-off-by: Andre Przywara <andre.przywara@arm.com>