arm64: dts: rockchip: use canonical compatible for touchpad/touchscreen on gru-kevin

"atmel,atmel_mxt_tp" and "atmel,atmel_mxt_ts" are ChromeOS inventions,
let's replace them with canonical compatible string "atmel,maxtouch".

Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Reviewed-by: Brian Norris <briannorris@chromium.org>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>