iio:adc:at91_adc8xx: introduce new atmel adc driver

This driver supports the new version of the Atmel ADC device introduced
with the SAMA5D2 SoC family.

Signed-off-by: Ludovic Desroches <ludovic.desroches@atmel.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>