dt-bindings: serial: sprd: Add dma properties to support DMA mode

  UART clock and source clock are optional properties, but enable clock

  is required.

Optional properties:

- dma-names: Should contain "rx" for receive and "tx" for transmit channels.

- dmas: A list of dma specifiers, one for each entry in dma-names.

Example:

	uart0: serial@0 {

		compatible = "sprd,sc9860-uart",

			     "sprd,sc9836-uart";

		reg = <0x0 0x100>;

		interrupts = <GIC_SPI 2 IRQ_TYPE_LEVEL_HIGH>;

		dma-names = "rx", "tx";

		dmas = <&ap_dma 19>, <&ap_dma 20>;

		clock-names = "enable", "uart", "source";

		clocks = <&clk_ap_apb_gates 9>, <&clk_uart0>, <&ext_26m>;

	};