clk: mediatek: export cpu multiplexer clock for MT8173 SoCs

The patch enables CPU multiplexer clock on MT8173 SoC which fixes up
cpufreq driver fails at acquiring intermediate clock source when driver
probe is called.

Signed-off-by: Pi-Cheng Chen <pi-cheng.chen@linaro.org>
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>