Fix the gpio-specifier decoding by respecting the GPIO controller's
#gpio-cells property.

Add a new ofw_bus method (OFW_BUS_MAP_GPIOS()) that allows the GPIO
controller to implement its own mapping to deal with gpio-specifiers,
allowing the decoding of gpio-specifiers to be controller specific.

The default ofw_bus_map_gpios() decodes the linux standard (#gpio-cells =
<2>) and the FreeBSD standard (#gpio-cells = <3>).

It pass the gpio-specifier flag field to the children as an ivar variable so
they can act upon.