ARM: dts: sun5i: Add touchscreen node to reference-design-tablet.dtsi

Just like on sun8i all sun5i tablets use the same interrupt and power
gpios for their touchscreens. I've checked all known a13 fex files and
only the UTOO P66 uses a different gpio for the interrupt.

Add a touchscreen node to sun5i-reference-design-tablet.dtsi, which
fills in the necessary gpios to avoid duplication in the tablet dts files,
just like we do in sun8i-reference-design-tablet.dtsi.

This will make future patches adding touchscreen nodes to a13 tablets
simpler.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>