zynqmp: Remove duplicate 'const' declaration

Fixing compilation errors due to duplicate 'const' keyword:
  plat/xilinx/zynqmp/pm_service/pm_client.c:39:29: error: duplicate 'const' declaration specifier [-Werror=duplicate-decl-specifier]
   static const struct pm_proc const pm_procs_all[] = {
                               ^~~~~

Signed-off-by: Soren Brinkmann <soren.brinkmann@xilinx.com>