mfd: max77620: Mask/unmask interrupt before/after servicing it

The programming guidelines of the MAX77620 for servicing interrupt is:
1. When interrupt occurs from PMIC, mask the PMIC interrupt by
   setting GLBLM.
2. Read IRQTOP and service the interrupt.
3. Once all interrupts has been checked and serviced, the interrupt
   service routine un-masks the hardware interrupt line by clearing
   GLBLM.

Add the pre and post interrupt service handler for mask and unmask the
global interrupt mask bit (for step 1 and 3) as callback from regmap-irq.

Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>