global: Move from bi_memstart/memsize -> gd->ram_base/ram_size

With the planned removal of bi_memstart & bi_memsize, this patch now
moves the references to the better suiting gd->ram_base/ram_size
variables.

Signed-off-by: Stefan Roese <sr@denx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>

arch/mips/lib/boot.c

@@ -17,7 +17,7 @@ unsigned long do_go_exec(ulong (*entry)(int, char * const []),
 	 * whole SDRAM area, since we don't know the size of the image
 	 * that was loaded.
 	 */
-	flush_cache(gd->bd->bi_memstart, gd->ram_top - gd->bd->bi_memstart);
+	flush_cache(gd->ram_base, gd->ram_top - gd->ram_base);
 
 	return entry(argc, argv);
 }