Documentation: of-serial.txt - update for clocks phandle for clk

	- "serial" if the port type is unknown.

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

- interrupts : should contain uart interrupt.

- clock-frequency : the input clock frequency for the UART.

- clock-frequency : the input clock frequency for the UART

	 or

  clocks phandle to refer to the clk used as per Documentation/devicetree

  /bindings/clock/clock-bindings.txt

Optional properties:

- current-speed : the current active speed of the UART.