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/common/cmd_fpga.c b/common/cmd_fpga.c
index 0ad310f..8946345 100644
--- a/common/cmd_fpga.c
+++ b/common/cmd_fpga.c
@@ -289,7 +289,7 @@
 			{
 				const void *fit_hdr = (const void *)fpga_data;
 				int noffset;
-				void *fit_data;
+				const void *fit_data;
 
 				if (fit_uname == NULL) {
 					puts ("No FIT subimage unit name\n");