pinctrl: qcom: Add irq_enable callback for msmgpio-dc

Introduce the irq_enable callback which will be same as irq_unmask
except that it will also clear the pending irq before unmask.

This will help in clearing any erroneous interrupts that would
have gotten latched when the interrupt is not in use.

Change-Id: Ie97b017a0731f1616845b9c54c3a6d3ba0d5a666
Signed-off-by: Maulik Shah <mkshah@codeaurora.org>