clk: rockchip: setup pll_mux data earlier

In some cases we might need to access the data of the pll mux before the actual
mux gets registered - like in the following patch adding an init-callback.

Therefore populate pll_mux before registering the core pll-clock.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>
Tested-by: Kever Yang <kever.yang@rock-chips.com>