Add support for old chips:
- Nuvoton NCT6116D
- and Winbond 83627DHG
but also for a more recent one:
- Nuvoton NCT6779
The refactoring help handle chips like the NCT6779 that span the control of a
given GPIO group across more than one logical device.
I modified gpioctl, en passant, to avoid listing invalid pins (unless verbose
is enabled). It help handle somewhat gracefully the Winbond 83627DHG having its
first group being GPIO2.
Successfully tested with Nuvoton NCT6116D and Nuvoton NCT6779.