gpio: qpnp-pin: map interrupts only when required

The current driver creates an irq mapping for every pin in the probe
function. This is unnecessary because some pins may not be used as
interrupts.

Instead create a mapping only when gpio_to_irq() api is called.

Change-Id: I8bd11997fbcbb0aad69ed0cfc20a2c35a8266f0f
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>