Add documentation for the mt8192 gce.

Add gce header file defined the gce hardware event,
subsys number and constant for mt8192.
Signed-off-by: Yongqiang Niu <yongqiang.niu@mediatek.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Hsin-Yi Wang <hsinyi@chromium.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>