ARM: dts: imx6dl: prtvt7: Remove touchscreen inversion

This touchscreen does not seem to be inverted in any axis, so remove these
DT properties.

Signed-off-by: David Jander <david@protonic.nl>
Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>