mailbox: mtk-cmdq: Fix local clock ID usage

In the probe function, the clock IDs were pointed to local variables
which should only be used in the same code block, and any access to them
after the probing stage becomes an use-after-free case.

Since there are only limited variants of the gce clock names so far, we
can just declare them as static constants to fix the issue.

Fixes: 85dfdbfc ("mailbox: cmdq: add multi-gce clocks support for mt8195")
Signed-off-by: Fei Shao <fshao@chromium.org>
Reviewed-by: Tzung-Bi Shih <tzungbi@google.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>