mailbox: mediatek: cmdq: clear the event in cmdq initial flow

GCE hardware stored event information in own internal sysram,
if the initial value in those sysram is not zero value
it will cause a situation that gce can wait the event immediately
after client ask gce to wait event but not really trigger the
corresponding hardware.

In order to make sure that the wait event function is
exactly correct, we need to clear the sysram value in
cmdq initial flow.

Fixes: 623a6143

 ("mailbox: mediatek: Add Mediatek CMDQ driver")

Signed-off-by: Bibby Hsieh <bibby.hsieh@mediatek.com>
Reviewed-by: CK Hu <ck.hu@mediatek.com>
Reviewed-by: Matthias Brugger <matthias.bgg@gmail.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>