Remove the properties that are not used anymore by the at91_adc driver and fix
the atmel,adc-use-external-triggers property name.

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