gpio: pch: allow use from device tree

Allow GPIOs from the gpio-pch driver to be referenced from device tree
by simply setting the struct gpio_chip of_node pointer to that of the
struct pci_dev.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>