ARM: dts: at91: sama5d4ek: use canonical compatible for touchscreen

The canonical compatible string is "atmel,maxtouch", let's use it. Also,
change the node name to be "touchscreen" rather than driver name.

Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>