drivers: gpio: Updating driver to Enable Multiple Interrupts.

Change Summary:
- Updating GPIO_CFG_IRQ define to enable multiple interrupts to
  accomodate for IP configuration where each pin when interrupted
  can trigger CPU interrupt.
- Removing gpio_dw_unmask_int define as it is NO OP

Signed-off-by: Ravik Hasija <ravikh@fb.com>