ARM: meson: documentation: Add timer documentation

Amlogic Meson6 SoCs Timer Controller

Required properties:

- compatible : should be "amlogic,meson6-timer"

- reg : Specifies base physical address and size of the registers.

- interrupts : The interrupt of the first timer

Example:

timer@c1109940 {

	compatible = "amlogic,meson6-timer";

	reg = <0xc1109940 0x14>;

	interrupts = <0 10 1>;

};