ARM: docs: add documentation binding for meson watchdog

Meson SoCs Watchdog timer

Required properties:

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

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

Example:

wdt: watchdog@c1109900 {

	compatible = "amlogic,meson6-wdt";

	reg = <0xc1109900 0x8>;

};