Documentation: dt: iio: at91-sama5d2_adc: add hw trigger edge binding

Add property for the edge type of the hardware trigger pin ADTRG

Signed-off-by: Eugen Hristev <eugen.hristev@microchip.com>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Ludovic Desroches <ludovic.desroches@microchip.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>