zynq: Use GPLed files for SPL

The latest Xilinx tools generate ps7_init files that are explicitly
available under GPL. Change the makefile to allow drop in of those files
for building the SPL.

Signed-off-by: Soren Brinkmann <soren.brinkmann@xilinx.com>
Acked-by: Jagannadha Sutradharudu Teki <jagannadh.teki@gmail.com>
Reviewed-and-tested-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
diff --git a/board/xilinx/zynq/xil_io.h b/board/xilinx/zynq/xil_io.h
index e59a977..1eccf8d 100644
--- a/board/xilinx/zynq/xil_io.h
+++ b/board/xilinx/zynq/xil_io.h
@@ -6,7 +6,7 @@
 #define XIL_IO_H
 
 /*
- * This empty file is here because ps7_init.c exported by hw project
+ * This empty file is here because ps7_init_gpl.c exported by hw project
  * has #include "xil_io.h" line.
  */