fpga: constify to fix build warning

Fix compiler warning:

cmd_fpga.c:318: warning: passing argument 3 of 'fit_image_get_data'
from incompatible pointer type

Adding the needed 'const' here entails a whole bunch of additonal
changes all over the FPGA code.

Signed-off-by: Wolfgang Denk <wd@denx.de>
Cc: Andre Schwarz <andre.schwarz@matrix-vision.de>
Cc: Murray Jensen <Murray.Jensen@csiro.au>
Acked-by: Andre Schwarz<andre.schwarz@matrix-vision.de>
diff --git a/include/spartan3.h b/include/spartan3.h
index 0f0b400..67ede4b 100644
--- a/include/spartan3.h
+++ b/include/spartan3.h
@@ -27,9 +27,9 @@
 
 #include <xilinx.h>
 
-extern int Spartan3_load( Xilinx_desc *desc, void *image, size_t size );
-extern int Spartan3_dump( Xilinx_desc *desc, void *buf, size_t bsize );
-extern int Spartan3_info( Xilinx_desc *desc );
+extern int Spartan3_load(Xilinx_desc *desc, const void *image, size_t size);
+extern int Spartan3_dump(Xilinx_desc *desc, const void *buf, size_t bsize);
+extern int Spartan3_info(Xilinx_desc *desc);
 
 /* Slave Parallel Implementation function table */
 typedef struct {