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>

arch/mips/ath79/setup.c

@@ -203,6 +203,8 @@ void __init plat_mem_setup(void)
 	fdt_start = fw_getenvl("fdt_start");
 	if (fdt_start)
 		__dt_setup_arch((void *)KSEG0ADDR(fdt_start));
+	else if (fw_arg0 == -2)
+		__dt_setup_arch((void *)KSEG0ADDR(fw_arg1));
 #ifdef CONFIG_BUILTIN_DTB
 	else
 		__dt_setup_arch(__dtb_start);