ASoC: dt-bindings: stm32: Ensure compatible pattern matches whole string

ASoC: dt-bindings: stm32: Ensure compatible pattern matches whole string

The compatible pattern "st,stm32-sai-sub-[ab]" is missing starting and
ending anchors, so any prefix and/or suffix would still be valid.

This also fixes a warning on the example:

Documentation/devicetree/bindings/sound/st,stm32-sai.example.dtb: /example-0/sai@4400b000/audio-controller@4400b004: failed to match any schema with compatible: ['st,stm32-sai-sub-a']

Signed-off-by: Rob Herring (Arm) <robh@kernel.org>
Link: https://msgid.link/r/20240520222705.1742367-1-robh@kernel.org
Signed-off-by: Mark Brown <broonie@kernel.org>