MIPS: Remove redundant definitions of device_tree_init()

There exists many same definitions of device_tree_init() for various
platforms, add a weak function in arch/mips/kernel/prom.c to clean
up the related code.

Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>