clk: k210: Move pll into the rest of the driver

Now that there no separate PLL driver, we can no longer make the PLL
functions static. By moving the PLL driver in with the rest of the clock
code, we can make these functions static again. We still keep the pll
header for unit testing, but it is pretty reduced.

Signed-off-by: Sean Anderson <seanga2@gmail.com>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>