dt-bindings: serial: sprd: Add clocks and clocks-names properties

- reg: offset and length of the register set for the device

- interrupts: exactly one interrupt specifier

- clocks: phandles to input clocks.

- clock-names: Should contain following entries:

  "enable" for UART module enable clock,

  "uart" for UART clock,

  "source" for UART source (parent) clock.

- clocks: Should contain a clock specifier for each entry in clock-names.

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

  is required.

Example:

	uart0: serial@0 {

			     "sprd,sc9836-uart";

		reg = <0x0 0x100>;

		interrupts = <GIC_SPI 2 IRQ_TYPE_LEVEL_HIGH>;

		clocks = <&ext_26m>;

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

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

	};