dt-bindings: serial: fix regex pattern for matching serial node children

The regular expression pattern for matching serial node children should
accept only nodes starting and ending with the set of words: bluetooth,
gnss, gps or mcu. Add missing brackets to enforce such matching.

Fixes: 0c559bc8abfb ("dt-bindings: serial: restrict possible child node names")
Reported-by: Andreas Kemnade <andreas@kemnade.info>
Closes: https://lore.kernel.org/all/20231004170021.36b32465@aktux/
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20231005093247.128166-1-krzysztof.kozlowski@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>