dt-bindings:iio:magnetometer:memsic,mmc35240: move to trivial-devices.yaml

Extremely simple binding so no need to maintain a separate file.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Cc: Jandy Gou <qingsong.gou@ck-telecom.com>
Link: https://lore.kernel.org/r/20201031182922.743153-7-jic23@kernel.org