ARM: dts: sunxi: Add sun?i-q8-common.dtsi files for q8 tablets

Cheap allwinner based devices in the q8 enclosure come in many variants,
all sharing the case and a number of other basic features.

They differ in the display, touchscreen, accelerometer and wifi chips
used.

This commit adds 2 dtsi files defining the shared features of all the
q8 tablets. sunxi-q8-common.dtsi defines features shared amongst all
q8 tablets, sun8i-q8-common.dtsi defines features shared amongst all
a23 / a33 based q8 tablets, but not with a13 q8 based tablets.

a13 based tablets use a different card-detect pin for the mmc, and
use uart1 instead of the r_uart for the serial console.

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