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: NHeiko Stuebner <heiko@sntech.de>
Reviewed-by: NKever Yang <kever.yang@rock-chips.com>
Tested-by: NKever Yang <kever.yang@rock-chips.com>