ARM: dts: sun8i: reference-design-tablet: Add gsl1680 touchscreen node

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>