ARM: dts: sunxi: sun6i-a31s-primo81.dts: add touchscreen axis swapping property

The MSI Primo81 has a display in portrait mode but a touchscreen
in landscape mode.  To have both of them use the same coordinate
system, the touchscreen-swapped-x-y property has to be set
for the touchscreen.

Signed-off-by: Karsten Merker <merker@debian.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>