MIPS: of: Introduce helper function to get DTB

Selection of the DTB to be used was burried in more or less readable
code in head.S. Move this code into a inline helper function and
use it.

Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>

arch/mips/generic/init.c

@@ -39,14 +39,13 @@ void __init *plat_get_fdt(void)
 		/* Already set up */
 		return (void *)fdt;
 
-	if (fw_passed_dtb && !fdt_check_header((void *)fw_passed_dtb)) {
+	fdt = (void *)get_fdt();
+	if (fdt && !fdt_check_header(fdt)) {
 		/*
 		 * We have been provided with the appropriate device tree for
 		 * the board. Make use of it & search for any machine struct
 		 * based upon the root compatible string.
 		 */
-		fdt = (void *)fw_passed_dtb;
-
 		for_each_mips_machine(check_mach) {
 			match = mips_machine_is_compatible(check_mach, fdt);
 			if (match) {