mfd: Allow to specify max77686 interrupt through DT or platform file also

Presently driver expects irq_gpio pin in platform data and maps it to irq
itself. But we can also directly specify the interrupt in DT or platform file.

Signed-off-by: Yadwinder Singh Brar <yadi.brar@samsung.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>