dt-bindings: mfd: Add Maxim MAX77714 PMIC

Add bindings for the MAX77714 PMIC with GPIO, RTC and watchdog.

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