iio: adc: ad7124: add 3db filter

This patch adds the LOW_PASS_FILTER_3DB_FREQUENCY attribute
in iio_chan_spec for each channel. The used filters are sinc3
or sinc4. The filter type with the highest output data rate
is used when setting a low pass frequency in the channel's sysfs.

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