staging: iio: resolver: ad2s1210: convert resolution to devicetree property

staging: iio: resolver: ad2s1210: convert resolution to devicetree property

Selecting the resolution was implemented as the `bits` sysfs attribute.
However, the selection of the resolution depends on how the hardware
is wired and the specific application, so this is rather a job for
devicetree to describe.

A new devicetree property `assigned-resolution-bits` to specify the
resolution required for each chip is added and the `bits` sysfs
attribute is removed.

Since the resolution is now supplied by a devicetree property, the
resolution-gpios are now optional and we can allow for the case where
the resolution pins on the AD2S1210 are hard-wired instead of requiring
them to be connected to gpios.

Signed-off-by: David Lechner <dlechner@baylibre.com>
Link: https://lore.kernel.org/r/20231005-ad2s1210-mainline-v4-4-ec00746840fc@baylibre.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>