ARM: pxa: use gpio_to_irq for sharppm_sl

Replace custom PXA_GPIO_TO_IRQ macro with standard gpio_to_irq.

Signed-off-by: Rob Herring <rob.herring@calxeda.com>
Cc: Eric Miao <eric.y.miao@gmail.com>
Cc: Haojian Zhuang <haojian.zhuang@gmail.com>