dt-bindings: timer: Drop unneeded quotes

Cleanup bindings dropping unneeded quotes. Once all these are fixed,
checking for this can be enabled in yamllint.

Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Thierry Reding <treding@nvidia.com>
Link: https://lore.kernel.org/r/20230327170146.4104556-1-robh@kernel.org


Signed-off-by: Rob Herring <robh@kernel.org>