clk: mediatek: add missing cpu mux causing Mediatek cpufreq can't work

This patch adds CPU multiplexer clocks which are essential for Mediatek
cpufreq driver. It would use the CPU clock multiplexer to switch to the
intermediate clock source temporarily and then wait for the primary clock
changing getting stable.

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>