arm: zynq: Add missing ps7_post_config declaration

Add missing declaration to header.

Warning log:
arch/arm/mach-zynq/spl.c:94:12: warning: symbol 'ps7_post_config' was
not declared. Should it be static?
Signed-off-by: NMichal Simek <michal.simek@xilinx.com>

arch/arm/mach-zynq/include/mach/sys_proto.h

@@ -22,5 +22,6 @@ int zynq_board_read_rom_ethaddr(unsigned char *ethaddr);
 
 /* Driver extern functions */
 extern void ps7_init(void);
+int ps7_post_config(void);
 
 #endif /* _SYS_PROTO_H_ */