owc_gpiobus: clean / fix up the driver module things

"fdt" is removed from the driver module name as the driver does not
require FDT and can work very well on hints based systems.

A module dependency is added for gpiobus. Without that owc cannot
resolve symbols in gpiobus if both are loaded as kernel modules.

Finally, a driver module module version is added.

Reviewed by: imp
MFC after: 11 days