staging: iio: resolver: ad2s1210: use devicetree to get CLKIN rate

staging: iio: resolver: ad2s1210: use devicetree to get CLKIN rate

This removes the fclkin sysfs attribute and replaces it with getting
the CLKIN clock rate using the clk subsystem (i.e. from the devicetree).

CLKIN comes from an external oscillator that is connected directly to
the AD2S1210 chip, so users of the sysfs attributes should not need to
be concerned with this.

The fclkin field (the datasheet name) is renamed to clkin_hz to be more
obvious that it is a frequency in Hz.

Signed-off-by: David Lechner <dlechner@baylibre.com>
Link: https://lore.kernel.org/r/20230929-ad2s1210-mainline-v3-9-fa4364281745@baylibre.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>