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

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

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>;

		...

	};