dt-bindings: watchdog: Add Meson GXBB Watchdog bindings

Meson GXBB SoCs Watchdog timer

Required properties:

- compatible : should be "amlogic,meson-gxbb-wdt"

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

- clocks : Should be a phandle to the Watchdog clock source, for GXBB the xtal

	   is the default clock source.

Example:

wdt: watchdog@98d0 {

	compatible = "amlogic,meson-gxbb-wdt";

	reg = <0 0x98d0 0x0 0x10>;

	clocks = <&xtal>;

};