iio: Add IIO_STATIC_CONST_DEVICE_ATTR

iio: Add IIO_STATIC_CONST_DEVICE_ATTR

Add IIO_STATIC_CONST_DEVICE_ATTR macro for creating an read-only
iio_dev_attr which returns constant value. This macro is intended to be
used when replacing the IIO_CONST_ATTR - attributes for triggered
buffers because the triggered buffer attributes must be of type
iio_dev_attr.

Signed-off-by: Matti Vaittinen <mazziesaccount@gmail.com>
Link: https://lore.kernel.org/r/8dd853dd0ef8eb40cb980cc6f6e7a43166de3afb.1664782676.git.mazziesaccount@gmail.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>