iio: expose shared parameter in IIO_ENUM_AVAILABLE

The shared parameter should be configurable based on its usage, and not
constrained to IIO_SHARED_BY_TYPE.

This patch aims to improve the flexibility in using the
IIO_ENUM_AVAILABLE define and avoid redefining custom iio enums that
expose the shared parameter.

An example is the ad5766.c driver where IIO_ENUM_AVAILABLE_SHARED was
defined in order to achieve `shared` parameter customization.

The current state of the IIO_ENUM_AVAILABLE implementation will imply
similar redefinitions each time a driver will require access to the
`shared` parameter. An example would be admv1013 driver which will
require custom device attribute for the frequency translation modes:
Quadrature I/Q mode and Intermediate Frequency mode.

Signed-off-by: Antoniu Miclaus <antoniu.miclaus@analog.com>
Reviewed-by: Alexandru Ardelean <ardeleanalex@gmail.com>
Link: https://lore.kernel.org/r/20211119085627.6348-1-antoniu.miclaus@analog.com


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