ASoC: dt-bindings: simple-card: document new system-clock-fixed flag

Document the new system-clock-fixed flag, which can be used to specify
that the driver cannot or should not allow the clock frequency of the
mapped clock to be modified.

Signed-off-by: Robert Hancock <robert.hancock@calian.com>
Link: https://lore.kernel.org/r/20220120195832.1742271-6-robert.hancock@calian.com


Signed-off-by: Mark Brown <broonie@kernel.org>