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/arch/powerpc/cpu/mpc8xx/fdt.c b/arch/powerpc/cpu/mpc8xx/fdt.c
index 4d952a3..226e258 100644
--- a/arch/powerpc/cpu/mpc8xx/fdt.c
+++ b/arch/powerpc/cpu/mpc8xx/fdt.c
@@ -25,5 +25,5 @@
 	do_fixup_by_compat_u32(blob, "fsl,cpm-brg", "clock-frequency",
 			       gd->arch.brg_clk, 1);
 
-	fdt_fixup_memory(blob, (u64)bd->bi_memstart, (u64)bd->bi_memsize);
+	fdt_fixup_memory(blob, (u64)gd->ram_base, (u64)gd->ram_size);
 }