Add documentation for the mt6779 gce.

Add gce header file defined the gce hardware event,
subsys number and constant for mt6779.
Signed-off-by: Dennis YC Hsieh <dennis-yc.hsieh@mediatek.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: CK Hu <ck.hu@mediatek.com>
Reviewed-by: Bibby Hsieh <bibby.hsieh@mediatek.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>