powerpc: Change free_bootmem() to kfree()

commit ea96025a('Don't use alloc_bootmem() in init_IRQ() path')
changed alloc_bootmem() to kzalloc(),
but missed to change free_bootmem() to kfree().
So correct it.

Signed-off-by: Joonsoo Kim <js1304@gmail.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>