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

Documentation/devicetree/bindings/sound/st,stm32-sai.yaml

@@ -68,7 +68,7 @@ patternProperties:
     properties:
       compatible:
         description: Compatible for SAI sub-block A or B.
-        pattern: "st,stm32-sai-sub-[ab]"
+        pattern: "^st,stm32-sai-sub-[ab]$"
 
       "#sound-dai-cells":
         const: 0