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 <bibby.hsieh@mediatek.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>