iio: adc: ti_am335x_adc: alloc kfifo & IRQ via devm_ functions

This change attaches the life-cycle of the kfifo buffer & IRQ to the
parent-device. This in turn cleans up the exit & error paths, since we
don't need to explicitly cleanup these resources.

The main intent here is to remove the explicit cleanup of the
'indio_dev->buffer' via 'iio_kfifo_free(indio_dev->buffer);'.

As we want to add support for multiple buffers per IIO device, having it
exposed like this makes it tricky to consider a safe backwards compatible
approach for it.

Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>