HID: wiimote: Parse nunchuck data

The Nintendo Nunchuck extension reports accelerometer values, one analog stick
and two buttons. See inline comments for data layout.
We report all data to userspace through extension input device.

Signed-off-by: David Herrmann <dh.herrmann@googlemail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>