qoriq-atf: fix array-bounds build failure

qoriq-atf: fix array-bounds build failure

Fix array-bounds build error:
common/runtime_svc.c: In function 'handle_runtime_svc':
common/runtime_svc.c:55:28: error: array subscript 'rt_svc_desc_t {aka const struct rt_svc_desc}[0]' is partly outside array bounds of 'uintptr_t[1]' {aka 'long unsigned int[1]'} [-Werror=array-bounds]
   55 |         return rt_svc_descs[index].handle(smc_fid, x1, x2, x3, x4, cookie,
      |                ~~~~~~~~~~~~^~~~~~~
In file included from common/runtime_svc.c:10:
include/common/runtime_svc.h:125:18: note: while referencing '__RT_SVC_DESCS_START__'
  125 | extern uintptr_t __RT_SVC_DESCS_START__;
      |                  ^~~~~~~~~~~~~~~~~~~~~~
  CC      lib/el3_runtime/aarch64/context_mgmt.c
cc1: all warnings being treated as errors

Signed-off-by: Ting Liu <ting.liu@nxp.com>