dt-binding: gce: add binding for gce client reg property

cmdq driver provide a function that get the relationship
of sub system number from device node for client.
add specification for #subsys-cells, mediatek,gce-client-reg.

Signed-off-by: Bibby Hsieh <[e-mail]>
Reviewed-by: Rob Herring <[e-mail]>
Signed-off-by: Jassi Brar <[e-mail]>

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

···
 Required properties for a client device:
 - mboxes: Client use mailbox to communicate with GCE, it should have this
   property and list of phandle, mailbox specifiers.
-- mediatek,gce-subsys: u32, specify the sub-system id which is corresponding
-  to the register address.
+Optional properties for a client device:
+- mediatek,gce-client-reg: Specify the sub-system id which is corresponding
+  to the register address, it should have this property and list of phandle,
+  sub-system specifiers.
+  <&phandle subsys_number start_offset size>
+  phandle: Label name of a gce node.
+  subsys_number: specify the sub-system id which is corresponding
+                 to the register address.
+  start_offset: the start offset of register address that GCE can access.
+  size: the total size of register address that GCE can access.
 
 Some vaules of properties are defined in 'dt-bindings/gce/mt8173-gce.h'
 or 'dt-binding/gce/mt8183-gce.h'. Such as sub-system ids, thread priority, event ids.
···
 		compatible = "mediatek,mt8173-mmsys";
 		mboxes = <&gce 0 CMDQ_THR_PRIO_LOWEST 1>,
 			 <&gce 1 CMDQ_THR_PRIO_LOWEST 1>;
-		mediatek,gce-subsys = <SUBSYS_1400XXXX>;
 		mutex-event-eof = <CMDQ_EVENT_MUTEX0_STREAM_EOF
 				CMDQ_EVENT_MUTEX1_STREAM_EOF>;
-
+		mediatek,gce-client-reg = <&gce SUBSYS_1400XXXX 0x3000 0x1000>,
+					  <&gce SUBSYS_1401XXXX 0x2000 0x100>;
 		...
 	};