ARM: dts: imx6: rename touch screen's node name to touchscreen

ARM: dts: imx6: rename touch screen's node name to touchscreen

Rename touch to touchscreen.
Rename stmpe_touchscreen to touchscreen.
Rename stmpe_adc to adc.

Fix below CHECK_DTBS warning:
arch/arm/boot/dts/nxp/imx/imx7d-nitrogen7.dtb: touch@48 (ti,tsc2004): 'wakeup-gpios' does not match any of the regexes: '^pinctrl-[0-9]+$'
        from schema $id: http://devicetree.org/schemas/input/touchscreen/ti,tsc2005.yaml#

Signed-off-by: Frank Li <Frank.Li@nxp.com>
Reviewed-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>