mfd: max77714: Add driver for Maxim MAX77714 PMIC

Add a simple driver for the Maxim MAX77714 PMIC, supporting RTC and
watchdog only.

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>