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/include/asm/prom.h

@@ -36,6 +36,17 @@ static inline unsigned long pci_address_to_pio(phys_addr_t address)
 }
 #define pci_address_to_pio pci_address_to_pio
 
+struct boot_param_header;
+
+extern void __dt_setup_arch(struct boot_param_header *bph);
+
+#define dt_setup_arch(sym)						\
+({									\
+	extern struct boot_param_header __dtb_##sym##_begin;		\
+									\
+	__dt_setup_arch(&__dtb_##sym##_begin);				\
+})
+
 #else /* CONFIG_OF */
 static inline void device_tree_init(void) { }
 #endif /* CONFIG_OF */

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;
+}