dt-bindings: mfd: Add DT bindings for max77650

Add a DT binding document for max77650 ultra-low power PMIC. This
describes the core mfd device and the GPIO module.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Lee Jones <lee.jones@linaro.org>