clk: mediatek: export cpu multiplexer clock for MT2701/MT7623 SoCs

The patch enables CPU multiplexer clock on MT2701/MT7623 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>