iio: st_sensors: Create extended attr macro

iio: st_sensors: Create extended attr macro

Extend ST_SENSORS_LSM_CHANNELS() to a version that will accept extended
attributes named ST_SENSORS_LSM_CHANNELS_EXT() and wrap the former as a
specialized version of the former.

Cc: Hans de Goede <hdegoede@redhat.com>
Cc: Denis Ciocca <denis.ciocca@st.com>
Cc: Daniel Drake <drake@endlessm.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Stephan Gerhold <stephan@gerhold.net>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Link: https://lore.kernel.org/r/20210518230722.522446-1-linus.walleij@linaro.org
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>