dt-bindings: iio: magnetometer: add dt-bindings for freescale mag3110

Add Freescale MAG3110 dt-bindings and remove it from trivial-devices
dt-bingding doc.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
[robh: fixup trivial-devices.txt change for trivial-devices.yaml]
Signed-off-by: Rob Herring <robh@kernel.org>