dt-bindings: iio: bmc150_magn: Document and fix missing compatibles

The driver supports also BMC156B and BMM150B.  Add existing compatibles
marking the BMM150B one as deprecated (due to redundant suffix "_magn"
because the device unlike two others is a magnetometer only).  Introduce
a new, proper compatible for the deprecated one.

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>