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/drivers/fpga/xilinx.c b/drivers/fpga/xilinx.c
index 08dfdec..4072cb4 100644
--- a/drivers/fpga/xilinx.c
+++ b/drivers/fpga/xilinx.c
@@ -48,7 +48,7 @@
 
 /* ------------------------------------------------------------------------- */
 
-int xilinx_load (Xilinx_desc * desc, void *buf, size_t bsize)
+int xilinx_load(Xilinx_desc *desc, const void *buf, size_t bsize)
 {
 	int ret_val = FPGA_FAIL;	/* assume a failure */
 
@@ -95,7 +95,7 @@
 	return ret_val;
 }
 
-int xilinx_dump (Xilinx_desc * desc, void *buf, size_t bsize)
+int xilinx_dump(Xilinx_desc *desc, const void *buf, size_t bsize)
 {
 	int ret_val = FPGA_FAIL;	/* assume a failure */