clk: sunxi-ng: h6: Reparent CPUX during PLL CPUX rate change

In the Linux kernel, the following vulnerability has been resolved:

clk: sunxi-ng: h6: Reparent CPUX during PLL CPUX rate change

While PLL CPUX clock rate change when CPU is running from it works in

vast majority of cases, now and then it causes instability. This leads

to system crashes and other undefined behaviour. After a lot of testing

(30+ hours) while also doing a lot of frequency switches, we can't

observe any instability issues anymore when doing reparenting to stable

clock like 24 MHz oscillator.