Add ofw_gpiobus_parse_gpios(), a new public function, to parse the gpios

Add ofw_gpiobus_parse_gpios(), a new public function, to parse the gpios
property for devices that doesn't descend directly from gpiobus.

The parser supports multiple pins, different GPIO controllers and can use
arbitrary names for the property (to match the many linux variants:
cd-gpios, power-gpios, wp-gpios, etc.).

Pass the driver name on ofw_gpiobus_add_fdt_child().  Update gpioled to
match.

An usage example of ofw_gpiobus_parse_gpios() will follow soon.