iio: multiplexer: Make use of device properties

Convert the module to be property provider agnostic and allow
it to be used on non-OF platforms.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Peter Rosin <peda@axentia.se>
Link: https://lore.kernel.org/r/20220302160025.54348-1-andriy.shevchenko@linux.intel.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>