iio: adc: at91-sama5d2_adc: add support for DMA

Added support for DMA transfers. The implementation uses the user watermark
to decide whether DMA will be used or not. For watermark 1, DMA will not be
used. If watermark is bigger, DMA will be used.
Sysfs attributes are created to indicate whether the DMA is used,
with hwfifo_enabled, and the current DMA watermark is readable
in hwfifo_watermark. Minimum and maximum values are in hwfifo_watermark_min
and hwfifo_watermark_max.
Signed-off-by: Eugen Hristev <eugen.hristev@microchip.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/adc/Kconfig

@@ -158,6 +158,7 @@ config AT91_SAMA5D2_ADC
 	tristate "Atmel AT91 SAMA5D2 ADC"
 	depends on ARCH_AT91 || COMPILE_TEST
 	depends on HAS_IOMEM
+	depends on HAS_DMA
 	select IIO_TRIGGERED_BUFFER
 	help
 	  Say yes here to build support for Atmel SAMA5D2 ADC which is