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: 0cd71145803d ("iio: st-sensors: Update ST Sensor bindings")
Signed-off-by: Sean Nyekjaer <[e-mail]>
Acked-by: Conor Dooley <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jonathan Cameron <[e-mail]>

Documentation/devicetree/bindings/iio/st,st-sensors.yaml

···
           - st,lsm9ds0-gyro
       - description: STMicroelectronics Magnetometers
         enum:
+          - st,iis2mdc
           - st,lis2mdl
           - st,lis3mdl-magn
           - st,lsm303agr-magn