MIPS: ralink: bootrom: mark a function as __init to save some memory

'bootrom_setup()' is only called via 'postcore_initcall'.
It can be marked as __init to save a few bytes of memory.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>