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

Documentation/devicetree/bindings/serial/samsung_uart.yaml

@@ -52,6 +52,16 @@ properties:
       - pattern: '^clk_uart_baud[0-3]$'
       - pattern: '^clk_uart_baud[0-3]$'
 
+  dmas:
+    items:
+      - description: DMA controller phandle and request line for RX
+      - description: DMA controller phandle and request line for TX
+
+  dma-names:
+    items:
+      - const: rx
+      - const: tx
+
   interrupts:
     description: RX interrupt and optionally TX interrupt.
     minItems: 1