Add support for generic hardware

It's essentially does the following:

1) Tests if gpio-admin is available. If it's not the case use direct export/unexport via sysfs instead.
2) Correctly handlel GPIOs without interrupt support
3) Adds pins_linux pinbank which is essentially the same as pins_broadcom_soc

I didn't tested it on Pi since I haven't got one but it should work there as before.