dt-bindings: mailbox: add definition for mt8186

Add definition of compatible and dt-binding header for mt8186.
Signed-off-by: Rex-BC Chen <rex-bc.chen@mediatek.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>

Documentation/devicetree/bindings/mailbox/mtk-gce.txt

@@ -10,7 +10,8 @@ mailbox.txt for generic information about mailbox device-tree bindings.
 
 Required properties:
 - compatible: can be "mediatek,mt8173-gce", "mediatek,mt8183-gce",
-  "mediatek,mt8192-gce", "mediatek,mt8195-gce" or "mediatek,mt6779-gce".
+  "mediatek,mt8186-gce", "mediatek,mt8192-gce", "mediatek,mt8195-gce" or
+  "mediatek,mt6779-gce".
 - reg: Address range of the GCE unit
 - interrupts: The interrupt signal from the GCE block
 - clock: Clocks according to the common clock binding
@@ -40,8 +41,9 @@ Optional properties for a client mutex node:
   defined in 'dt-bindings/gce/<chip>-gce.h'.
 
 Some vaules of properties are defined in 'dt-bindings/gce/mt8173-gce.h',
-'dt-bindings/gce/mt8183-gce.h', 'dt-bindings/gce/mt8192-gce.h',
-'dt-bindings/gce/mt8195-gce.h' or 'dt-bindings/gce/mt6779-gce.h'.
+'dt-bindings/gce/mt8183-gce.h', 'dt-bindings/gce/mt8186-gce.h'
+'dt-bindings/gce/mt8192-gce.h', 'dt-bindings/gce/mt8195-gce.h' or
+'dt-bindings/gce/mt6779-gce.h'.
 Such as sub-system ids, thread priority, event ids.
 
 Example: