dt-bindings: yamllint: Enable quoted string check

yamllint has a quoted string check. Enable the check aligned with the
DT schema style which is only using quotes when necessary with the
exception of regex patterns. There's also the frequent occurrence of '/'
which we allow rather than fixing.

Link: https://lore.kernel.org/all/20230324202243.2442956-1-robh@kernel.org/Signed-off-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/.yamllint

 extends: relaxed
 
 rules:
+  quoted-strings:
+    required: only-when-needed
+    extra-allowed:
+      - '[$^,[]'
+      - '^/$'
   line-length:
     # 80 chars should be enough, but don't fail if a line is longer
     max: 110