MIPS: Lantiq: Add support for device tree file from boot loader

This fetches the device tree file like it is specified in the MIPS UHI
interface if one was found. This is also used when the device tree file
was appended to the kernel image with cat.
This code is copied from arch/mips/bmips/setup.c.
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Acked-by: John Crispin <john@phrozen.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/12898/Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/lantiq/prom.c

@@ -65,6 +65,8 @@ static void __init prom_init_cmdline(void)
 
 void __init plat_mem_setup(void)
 {
+	void *dtb;
+
 	ioport_resource.start = IOPORT_RESOURCE_START;
 	ioport_resource.end = IOPORT_RESOURCE_END;
 	iomem_resource.start = IOMEM_RESOURCE_START;
@@ -72,11 +74,18 @@ void __init plat_mem_setup(void)
 
 	set_io_port_base((unsigned long) KSEG1);
 
+	if (fw_arg0 == -2) /* UHI interface */
+		dtb = (void *)fw_arg1;
+	else if (__dtb_start != __dtb_end)
+		dtb = (void *)__dtb_start;
+	else
+		panic("no dtb found");
+
 	/*
-	 * Load the builtin devicetree. This causes the chosen node to be
+	 * Load the devicetree. This causes the chosen node to be
 	 * parsed resulting in our memory appearing
 	 */
-	__dt_setup_arch(__dtb_start);
+	__dt_setup_arch(dtb);
 }
 
 void __init device_tree_init(void)