MIPS: ralink: annotate prom_soc_init() with __init

prom_soc_init is only called from prom_init in arch/mips/ralink/prom.c,
which is already annotated with __init, so annotate prom_soc_init with
__init too.

Signed-off-by: Ilya Lipnitskiy <ilya.lipnitskiy@gmail.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>