iio: magnetometer: st_magn: Add LSM303C

The magnetometer part of ST LSM303C is similar (perhaps even identical)
to the already supported standalone LIS3MDL magnetometer, so just
add the new st,lsm303c-magn compatible for the existing definitions.

Signed-off-by: Stephan Gerhold <stephan@gerhold.net>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20230106102239.9647-4-stephan@gerhold.net


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