Rewrite gpioiic(4) to use the gpio_pin_* API, and to conform to the modern
FDT bindings document for gpio-i2c devices.

Using the gpio_pin_* functions to acquire/release/manipulate gpio pins
removes the constraint that both gpio pins must belong to the same gpio
controller/bank, and that the gpioiic instance must be a child of gpiobus.
Removing those constraints allows the driver to be fully compatible with
the modern dts bindings for a gpio bitbanged i2c bus.

For hinted attachment, the two gpio pins still must be on the same gpiobus,
and the device instance must be a child of that bus. This preserves
compatibility for existing installations that have use gpioiic(4) with hints.