pinctrl: coh901: add device tree support

This makes it possible to probe the COH901 pinctrl driver from
the device tree, and assigned the device tree node in the
gpio_chip so we can look up cross-references from the device
tree. Start grabbing the per-port (bank) IRQs by index instead
of by name so we don't have to look up the IRQs by name going
forward.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>