iio: resolver: ad2s1210: add support for adi,fixed-mode

iio: resolver: ad2s1210: add support for adi,fixed-mode

It is possible to use the AD2S1210 with hardwired mode pins (A0 and A1).
According to the devicetree bindings, in this case the adi,fixed-mode
property will specify which of the 3 possible modes the mode pins are
hardwired for and the gpio-modes property is not allowed.

This adds support for the case where the mode pins are hardwired for
config mode. In this configuration, the position and value must be read
from the config register.

The case of hardwired position or velocity mode is not supported as
there would be no way to configure the device.

Signed-off-by: David Lechner <dlechner@baylibre.com>
Reviewed-by: Nuno Sa <nuno.sa@analog.com>
Link: https://lore.kernel.org/r/20231016135423.16808-1-dlechner@baylibre.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>