dt-bindings:iio:imu:st,lsm6dsx: txt to yaml conversion

Straight forward conversion, but there are a few generic properties
in here like wakeup-source which should probably have schema in a
more generic location.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Cc: Lorenzo Bianconi <lorenzo@kernel.org>
Link: https://lore.kernel.org/r/20201031184854.745828-13-jic23@kernel.org