rtc: max77686: Use REGMAP_IRQ_REG for regmap-rtc-irqs initialisation

Use macro REGMAP_IRQ_REG from regmap.h to initialise the
regmap irq table for max77686 to have better coding style
and improve readability.

Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
Reviewed-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>