dt-bindings: serial: samsung: include generic dtschema to match bluetooth child

Include the generic serial.yaml dtschema so the child node like
"bluetooth" will be properly matched:

  arch/arm/boot/dts/exynos4210-universal_c210.dt.yaml:
    serial@13800000: 'bluetooth' does not match any of the regexes: 'pinctrl-[0-9]+'

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Link: https://lore.kernel.org/r/20210409172027.11296-1-krzysztof.kozlowski@canonical.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>