ARM: dts: at91: Fix ADC trigger nodes

The triggers don't need a reg property, remove it when prenset. Also remove
the unit-address from their name.

This solves:
Warning (unit_address_vs_reg): Node /ahb/apb/adc@fc034000/trigger@0 has a unit name, but no reg property
Warning (unit_address_vs_reg): Node /ahb/apb/adc@fc034000/trigger@1 has a unit name, but no reg property
Warning (unit_address_vs_reg): Node /ahb/apb/adc@fc034000/trigger@2 has a unit name, but no reg property
Warning (unit_address_vs_reg): Node /ahb/apb/adc@fc034000/trigger@3 has a unit name, but no reg property

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>