iio: magnetometer: add support for QST QMC6308

The QST QMC6308 is a 3-axis anisotropic magnetoresistive (AMR)
magnetometer on I2C, found e.g. in the Fairphone 6. Its register map
is not compatible with the QMC5883L's, so add a separate driver rather
than extending the QMC5883L driver.

Support raw X/Y/Z reads, output data rate, field range, filter
oversampling ratio, the mount matrix, and runtime PM.

Run measurements in the chip's periodic "normal" mode paced by the
DRDY flag rather than in its one-shot "single" mode: the datasheet
specifies no conversion time that could bound a one-shot wait, while
normal mode is paced by the specified output data rates, which also
keeps the sampling_frequency ABI meaningful.

Runtime PM puts the chip into its suspend mode after 500 ms without a
reading, dropping supply current to a few microamps. The suspended
chip retains its registers and keeps responding on I2C, so resuming
only rewrites the mode field and discards one stale sample, and
configuration changes apply even while suspended; VDD is left enabled
across runtime suspend.

Assisted-by: Claude:claude-fable-5
Signed-off-by: Jorijn van der Graaf <jorijnvdgraaf@catcrafts.net>
Signed-off-by: Jonathan Cameron <jonathan.cameron@oss.qualcomm.com>

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