Remove the properties that are not used anymore by the at91_adc driver.

Also, add #address-cells, #size-cells and a reg for each trigger to comply to
the ePAPR.
Signed-off-by: NAlexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: NNicolas Ferre <nicolas.ferre@atmel.com>