iio: adc: at91_adc: remove platform data and move defs in driver file

The AT91 ADC driver no longer uses the 'at91_add_device_adc' platform data
type. This is no longer used (at least in mainline boards).

This change removes the platform-data initialization from the driver, since
it is mostly dead code now.

Some definitions [from the platform data at91_adc.h include] have been
moved in the driver, since they are needed in the driver.

Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Reviewed-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20200930135048.11530-5-alexandru.ardelean@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>