clk: rockchip: don't use clk_ APIs in the pll init-callback

Separate the update of pll registers from the actual set_rate function
so that the init callback does not need to access clk-API functions.

As we now have separated the getting and setting of the pll parameters
we can also directly use these new functions in other places too.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>