HID: lenovo: Add support for Thinkpad X1 Tablet Thin keyboard

The Thinkpad X1 Tablet Thin keyboard's HID interface for the media-keys
and other special functions, is quite similar to the Thinkpad 10 ultrabook
keyboard's mouse/media-keys HID interface.

The only difference is that it needs a bit different key mappings.

Add support for the mute-LED and the non-standard media-keys on this
keyboard by re-using the tp10_ultrabook_kbd code combined with a new
lenovo_input_mapping_x1_tab_kbd() function.

Co-authored-by: Alexander Kobel <a-kobel@a-kobel.de>
Tested-by: Alexander Kobel <a-kobel@a-kobel.de>
Signed-off-by: Alexander Kobel <a-kobel@a-kobel.de>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>