Almost all sun8i tablets use a gsl1680 touchscreen controller, so add
a node describing it to reference-design-tablet.dtsi.
Almost all, but not all, so mark it as disabled by default (dts files
will also need to supply a firmware-name and resolution info).
Signed-off-by: Hans de Goede <hdegoede@redhat.com> Acked-by: Chen-Yu Tsai <wens@csie.org> Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>