rtc: max77686: fail to probe if no RTC regmap irqchip is set

The max77686 mfd driver adds a regmap IRQ chip which creates an IRQ
domain that is used to map the virtual RTC alarm1 interrupt.

The RTC driver assumes that this will always be true since the PMIC IRQ
is a required property according to the max77686 DT binding doc.  If an
"interrupts" property is not defined for a max77686 PMIC, then the mfd
probe function will fail and the RTC platform driver will never be
probed.

But even when it is not possible to probe the rtc-max77686 driver
without a regmap IRQ chip, it's better to explicitly check if the IRQ
chip data is not NULL and gracefully fail instead of getting an OOPS.

Signed-off-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Reported-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Cc: Doug Anderson <dianders@chromium.org>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Cc: Olof Johansson <olof@lixom.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>