dt-bindings: iio: st-sensors: Re-add IIS2MDC magnetometer

"iio: st-sensors: Update ST Sensor bindings" accidentially dropped
the compatible for the IIS2MDC magnetometer.

Fixes: 0cd71145 ("iio: st-sensors: Update ST Sensor bindings")
Signed-off-by: Sean Nyekjaer <sean@geanix.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://patch.msgid.link/20241129-stmagdt-v1-1-963f0347fb0a@geanix.com


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