Documentation: watchdog: Add new bindings for meson8b

We make the documentation not meson6 specific anymore and add the new
binding for meson8b SoC.
Signed-off-by: Carlo Caione <carlo@endlessm.com>

Documentation/devicetree/bindings/watchdog/meson6-wdt.txt → Documentation/devicetree/bindings/watchdog/meson-wdt.txt

@@ -2,7 +2,7 @@ Meson SoCs Watchdog timer
 
 Required properties:
 
-- compatible : should be "amlogic,meson6-wdt"
+- compatible : should be "amlogic,meson6-wdt" or "amlogic,meson8b-wdt"
 - reg : Specifies base physical address and size of the registers.
 
 Example: