gpio: lpc32xx: 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: Roland Stigge <stigge@antcom.de>
Acked-by: Vladimir Zapolskiy <vz@mleia.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>