HID: Add new Letsketch tablet driver

Add a new driver for the LetSketch / VSON WP9620N drawing tablet. This
drawing tablet is also sold under other brand names such as Case U,
presumably this driver will work for all of them. But it has only been
tested with a LetSketch WP9620N model.

These tablets also work without a special HID driver, but then only
part of the active area works and both the pad and stylus buttons are
hardwired to special key-combos. E.g. the 2 stylus buttons send right
mouse clicks / resp. "e" key presses.

BugLink: https://bugzilla.redhat.com/show_bug.cgi?id=2005575
BugLink: https://github.com/DIGImend/digimend-kernel-drivers/issues/528


Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>