pinctrl: meson: use gpiochip data pointer

This makes the driver use the data pointer added to the gpio_chip
to store a pointer to the state container instead of relying on
container_of().

Cc: Beniamino Galvani <b.galvani@gmail.com>
Cc: Carlo Caione <carlo@endlessm.com>
Cc: Antoine Tenart <antoine.tenart@free-electrons.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>