zynqmp: Set RESET_TO_BL31 through platform.mk
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 <f9793267bd24439886cd4a142f9642a9bcc6364b@xilinx.com>

sbranden/arm-trusted-firmware,achingupta/arm-trusted-firmware,lsigithub/arm-trusted-firmware_public,lsigithub/arm-trusted-firmware_public,achingupta/arm-trusted-firmware,sbranden/arm-trusted-firmware