gpio_acpi: Introduce separate setup_intr and teardown_intr functions.

gpio_acpi: Introduce separate setup_intr and teardown_intr functions.

* This allows us to actually reserve the gpio pin before, and separately
  from actually setting up the interrupt handler.

* XXX This interface is still a bit inconvenient to use, because the
  code allows gpio_setup_intr to fail. Instead the gpio_alloc_intr
  function should be returning a cookie to use for gpio_setup_intr
  (which could obviously never fail then).