[ARM] Fix external DTB loading at bootup time

The patch "[ARM] Zero-out bss section at booting time" with
commit ab4688ba40ddc417f1e5b0d6cd8afd9b895d584e corrupts the
address of externally provided DTB via bootloader.

This patch fixes this issue by explicity loading address of
external DTB from _boot_reg2.

Signed-off-by: Anup Patel <anup@brainfault.org>