mfd: max77693: Update DT binding to support haptic

This patch add haptic DT binding documentation and example
to support haptic driver in max77693 Multifunction device.

Signed-off-by: Jaewon Kim <jaewon02.kim@samsung.com>
Acked-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>