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>
diff --git a/cmd/bedbug.c b/cmd/bedbug.c
index 81ce256..684e4a9 100644
--- a/cmd/bedbug.c
+++ b/cmd/bedbug.c
@@ -348,7 +348,7 @@
 		return 1;
 	}
 
-	top = gd->bd->bi_memstart + gd->bd->bi_memsize;
+	top = gd->ram_start + gd->ram_size;
 	depth = 0;
 
 	printf ("Depth     PC\n");