x86/gpio: Implement x86 gpio_to_irq convert function

We need this for x86 MID platforms where GPIO interrupts are
used. No special magic is needed so the default 1:1 behaviour
will do nicely.

Signed-off-by: Alek Du <alek.du@intel.com>
Signed-off-by: Jacob Pan <jacob.jun.pan@intel.com>
Signed-off-by: Alan Cox <alan@linux.intel.com>
LKML-Reference: <20110111105439.24448.69863.stgit@bob.linux.org.uk>
Signed-off-by: Ingo Molnar <mingo@elte.hu>