zynqmp: Set RESET_TO_BL31 through platform.mk

ZynqMP only supports builds with RESET_TO_BL31=1. Set this option
through the platform makefile on default.
Signed-off-by: Soren Brinkmann <soren.brinkmann@xilinx.com>

plat/xilinx/zynqmp/platform.mk

@@ -31,6 +31,7 @@ PROGRAMMABLE_RESET_ADDRESS := 1
 PSCI_EXTENDED_STATE_ID := 1
 A53_DISABLE_NON_TEMPORAL_HINT := 0
 SEPARATE_CODE_AND_RODATA := 1
+RESET_TO_BL31 := 1
 
 ifdef ZYNQMP_ATF_MEM_BASE
     $(eval $(call add_define,ZYNQMP_ATF_MEM_BASE))