dt-bindings: iio: adc: atmel,sama5d2-adc: make atmel,trigger-edge-type non-mandatory

dt-bindings: iio: adc: atmel,sama5d2-adc: make atmel,trigger-edge-type non-mandatory

The atmel,trigger-edge-type was never imposed by the driver.
Make things right and remove this property from the mandatory list.
This will not break existing nodes because according to the binding they
should have this property.
However, the driver does not impose it and it works without it, the property
selects the trigger type, and without it, the driver will have no trigger
available, which is the case on some boards which do not have access
to the trigger pin.
This will avoid generating this warning for example:

*/arch/arm/boot/dts/at91-sama7g5ek.dt.yaml: adc@e1000000: 'atmel,trigger-edge-type' is a required property
From schema: */Documentation/devicetree/bindings/iio/adc/atmel,sama5d2-adc.yaml

Signed-off-by: Eugen Hristev <eugen.hristev@microchip.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20211217095401.583821-1-eugen.hristev@microchip.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>