mfd: max77693: Handle IRQs using regmap

This patch modifies mfd driver to use regmap for handling interrupts.
It allows to simplify irq handling process. This modifications needed
to make small changes in function drivers, which use interrupts.

Signed-off-by: Robert Baldyga <r.baldyga@samsung.com>
Reviewed-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Acked-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>