mfd: max77686: Add power management support

The driver doesn't have PM operations defined so add a suspend
and resume function handlers to allow the PMIC IRQ to wakeup
the system when it is put into a sleep state.

Signed-off-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Reviewed-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>