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

The files ps7_init.c and ps7_init.h are supposed to be generated by
hw projects such as Vivado, PlanAhead and then to be copied into
board/xilinx/zynq directory.

But some prototypes in them cause annoying warning messages:

  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]

The prototypes should be

  int ps7_init(void);
  int ps7_post_config(void);
  int ps7_debug(void);

rather than

  int ps7_init();
  int ps7_post_config();
  int ps7_debug();

We do not want to be bothered because of automatically generated files.
But we cannot touch the external projects for now.
What we can do is to disable -Wstrict-prototypes for ps7_init.c

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

board/xilinx/zynq/Makefile

@@ -10,3 +10,6 @@ obj-y	:= board.o
 # Please copy ps7_init.c/h from hw project to this directory
 obj-$(CONFIG_SPL_BUILD) += \
 		$(if $(wildcard $(srctree)/$(src)/ps7_init.c), ps7_init.o)
+
+# Suppress "warning: function declaration isn't a prototype"
+CFLAGS_REMOVE_ps7_init.o := -Wstrict-prototypes