MIPS: head.S: Init fw_passed_dtb to builtin DTB

Init the 'fw_passed_dtb' pointer to the buit-in Device Tree blob when it
has been compiled in with CONFIG_BUILTIN_DTB.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>