x86: add irq_cfg in io_apic_64.c

preallocate size is 32, and if it is not enough, irq_cfg will more
via alloc_bootmem() or kzalloc(). (depending on how early we are in
system setup)

v2: fix typo about size of init_one_irq_cfg ... should use sizeof(struct irq_cfg)
v3: according to Eric, change get_irq_cfg() to irq_cfg()
v4: squash add irq_cfg_alloc in
Signed-off-by: Yinghai Lu <yhlu.kernel@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>