serial: sh-sci: Add DMA support to the DT binding documentation

Note: Each enabled SCIx UART should have an alias correctly numbered in the

"aliases" node.

Optional properties:

  - dmas: Must contain a list of two references to DMA specifiers, one for

	  transmission, and one for reception.

  - dma-names: Must contain a list of two DMA names, "tx" and "rx".

Example:

	aliases {

		serial0 = &scifa0;

		interrupts = <0 144 IRQ_TYPE_LEVEL_HIGH>;

		clocks = <&mstp2_clks R8A7790_CLK_SCIFA0>;

		clock-names = "sci_ick";

		dmas = <&dmac0 0x21>, <&dmac0 0x22>;

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

	};