staging: iio: resolver: ad2s1210: convert fexcit to channel attribute

staging: iio: resolver: ad2s1210: convert fexcit to channel attribute

The ad2s1210 driver has a device-specific attribute `fexcit` for setting
the frequency of the excitation output. This converts it to a channel in
order to use standard IIO ABI.

The excitation frequency is an analog output that generates a sine wave.
Only the frequency is configurable. According to the datasheet, the
specified range of the excitation frequency is from 2 kHz to 20 kHz and
can be set in increments of 250 Hz.

Signed-off-by: David Lechner <dlechner@baylibre.com>
Link: https://lore.kernel.org/r/20231005-ad2s1210-mainline-v4-3-ec00746840fc@baylibre.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>