x86: put irq_2_pin pointer into irq_cfg

preallocate 32 irq_2_pin entries, and use get_one_free_irq_2_pin() to get
one more and link to irq_cfg if needed.

so don't waste one where no irq is enabled.

Signed-off-by: Yinghai Lu <yhlu.kernel@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>