MIPS: Add helper function to allow platforms to point at a DTB.

Add __dt_setup_arch() that can be called to load a builtin DT.
Additionally we add a macro to allow loading a specific symbol
from the __dtb_* section.

Signed-off-by: John Crispin <blogic@openwrt.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/3715/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/kernel/prom.c

@@ -95,3 +95,14 @@ void __init device_tree_init(void)
 	/* free the space reserved for the dt blob */
 	free_mem_mach(base, size);
 }
+
+void __init __dt_setup_arch(struct boot_param_header *bph)
+{
+	if (be32_to_cpu(bph->magic) != OF_DT_HEADER) {
+		pr_err("DTB has bad magic, ignoring builtin OF DTB\n");
+
+		return;
+	}
+
+	initial_boot_params = bph;
+}