Pass device tree node as a part of gpio_pin_get_by_ofw_XXX API

Current API assumes that "gpios" property belongs to the device's node but for
some binding it's not true: gpiokeys has set of child nodes with this property.

Patch adds new argument instead of replacing device_t because device_t will be
used to track ownership for allocated pins

Reviewed by: mmel
Differential Revision: https://reviews.freebsd.org/D6277