dt-bindings: iio: st,st-sensors add IIS2MDC.

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

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: LI Qingwu <Qing-wu.Li@leica-geosystems.com.cn>
Link: https://lore.kernel.org/r/20210317063902.19300-2-Qing-wu.Li@leica-geosystems.com.cn


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