[MIPS] Mark prom_free_prom_memory as __init_refok

> WARNING: vmlinux.o(.text+0xbf20): Section mismatch: reference to
> .init.text:prom_free_prom_memory (between 'free_initmem' and 'copy_from_user_page')

prom_free_prom_memory() is called _before_ freeing init sections, so
it is false positive.  __init_refok can be used for such cases.

Signed-off-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>