MIPS: ath79: Add support for DTB passed using the UHI boot protocol

MIPS: ath79: Add support for DTB passed using the UHI boot protocol

This is needed for bootloader supporting UHI and to support appended
DTB.

Signed-off-by: Alban Bedel <albeu@free.fr>
Cc: Felix Fietkau <nbd@openwrt.org>
Cc: Antony Pavlov <antonynpavlov@gmail.com>
Cc: Gabor Juhos <juhosg@openwrt.org>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/12230/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>