gpio-cdev: add nu801 userspace driver

This adds a userspace interpretation of the nu801 driver used by Meraki
hardware. Previously this was a driver that was added per target, but as
multiple targets now have this driver, we should move to something that
can be shared by all targets since no driver exists upstream.

Co-developed-by: Christian Lamparter <chunkeey@gmail.com>
Signed-off-by: Chris Blake <chrisrblake93@gmail.com>
Signed-off-by: Christian Lamparter <chunkeey@gmail.com>