dt-bindings: mfd: da9062: add gpio bindings

Add gpio device documentation to make the da9062 gpios available for
users.

Signed-off-by: Marco Felsch <m.felsch@pengutronix.de>
Reviewed-by: Adam Thomson <Adam.Thomson.Opensource@diasemi.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>