ARM: dts: imx6: skov: migrate to resistive-adc-touch

Start to use more generic resistive-adc-touch node. ti,tsc2046 is an ADC
controller used for resistive touchscreen. So, define it as ADC and
attach resistive-adc-touch to it.

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