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/mips/lib/bootm.c b/arch/mips/lib/bootm.c
index 0a13f6e..d5c99d8 100644
--- a/arch/mips/lib/bootm.c
+++ b/arch/mips/lib/bootm.c
@@ -242,7 +242,7 @@
 #if CONFIG_IS_ENABLED(MIPS_BOOT_FDT) && CONFIG_IS_ENABLED(OF_LIBFDT)
 int arch_fixup_fdt(void *blob)
 {
-	u64 mem_start = virt_to_phys((void *)gd->bd->bi_memstart);
+	u64 mem_start = virt_to_phys((void *)gd->ram_base);
 	u64 mem_size = gd->ram_size;
 
 	return fdt_fixup_memory_banks(blob, &mem_start, &mem_size, 1);