ARM: at91/dt: sam9g45: improve ADC/touchscreen support

Fixes the compatible string, adds the pinmuxing for the ADC pins.

Also, removes atmel,adc-use-external-triggers as it is not possible to remove it
unless redefining the whole adc node

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>