iio: adc: at91_adc: rework trigger definition

Move the available trigger definition back in the driver to stop cluttering
the device tree. There is no functional change except that it actually
fixes the available triggers for at91sam9rl as it inherited the list from
at91sam9260 but actually has the triggers from at91sam9x5.

Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Reviewed-by: Ludovic Desroches <ludovic.desroches@microchip.com>
Link: https://lore.kernel.org/r/20201128222818.1910764-6-alexandre.belloni@bootlin.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>