zynq: disable -Wstrict-prototypes option for ps7_init.c

ps7_init.c/ps7_init.h

  CC      spl/board/xilinx/zynq/ps7_init.o
In file included from board/xilinx/zynq/ps7_init.c:50:0:
board/xilinx/zynq/ps7_init.h:137:1: warning: function declaration isn't a prototype [-Wstrict-prototypes]
board/xilinx/zynq/ps7_init.h:138:1: warning: function declaration isn't a prototype [-Wstrict-prototypes]
board/xilinx/zynq/ps7_init.h:139:1: warning: function declaration isn't a prototype [-Wstrict-prototypes]
board/xilinx/zynq/ps7_init.h:145:1: warning: function declaration isn't a prototype [-Wstrict-prototypes]
board/xilinx/zynq/ps7_init.c:12602:1: warning: function declaration isn't a prototype [-Wstrict-prototypes]
board/xilinx/zynq/ps7_init.c:12723:1: warning: function declaration isn't a prototype [-Wstrict-prototypes]
board/xilinx/zynq/ps7_init.c:12742:1: warning: function declaration isn't a prototype [-Wstrict-prototypes]
board/xilinx/zynq/ps7_init.c:12761:1: warning: function declaration isn't a prototype [-Wstrict-prototypes]
board/xilinx/zynq/ps7_init.c:12854:6: warning: function declaration isn't a prototype [-Wstrict-prototypes]

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Cc: Michal Simek <michal.simek@xilinx.com>