zynqmp: remove RESET_TO_BL31=1 from build instruction

RESET_TO_BL31=1 is specified by plat/xilinx/zynqmp/platform.mk with
"override" directive.  So, RESET_TO_BL31=1 is guaranteed without any
operation on users' side.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>