MIPS: Create a helper function for DT setup

A couple of platforms register two buses and call of_platform_populate().
Move this into a common function to reduce duplication.

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
Cc: f.fainelli@gmail.com
Cc: mbizon@freebox.fr
Cc: jogo@openwrt.org
Cc: jfraser@broadcom.com
Cc: linux-mips@linux-mips.org
Cc: devicetree@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/8167/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/include/asm/prom.h

@@ -22,6 +22,7 @@ extern void device_tree_init(void);
 struct boot_param_header;
 
 extern void __dt_setup_arch(void *bph);
+extern int __dt_register_buses(const char *bus0, const char *bus1);
 
 #define dt_setup_arch(sym)						\
 ({									\