Pinctrl nodes adhering to the generic pinctrl bindings may contain
properties like input-low/input-high, which require pinctrl_ops.set_state
to have a handle on the relevant gpio chip.

Currently this would lead to code duplication: Both the pinctrl driver
and gpiolib will need to store a list of registered gpiochips.

Avoid this by providing a helper to query the registered gpio_chip
given a gpio's number.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>