iio:magnetometer: Add Support for ST IIS2MDC

Add support for ST magnetometer IIS2MDC,
an I2C/SPI interface 3-axis magnetometer.
The patch was tested on the instrument with IIS2MDC via I2C interface.

Signed-off-by: LI Qingwu <Qing-wu.Li@leica-geosystems.com.cn>
Link: https://lore.kernel.org/r/20210317063902.19300-3-Qing-wu.Li@leica-geosystems.com.cn


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