dt-bindings: serial: samsung: add DMA properties

dt-bindings: serial: samsung: add DMA properties

The Samsung SoC UART nodes have usually DMA so dtschema has to reflect
this to fix dtbs_check warnings like:

  arch/arm/boot/dts/exynos4210-smdkv310.dt.yaml: serial@13800000:
    'dma-names', 'dmas' do not match any of the regexes: 'pinctrl-[0-9]+'

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210212163905.70171-1-krzk@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>