An RCG may act as a mux that switch between 2 parents.
This is the case on IPQ6018 and IPQ8074 where the APCS core clk that feeds
the CPU cluster clock just switches between XO and the PLL that feeds it.

Add the required ops to add support for this special configuration and use
the generic mux function to determine the rate.

This way we dont have to keep a essentially dummy frequency table to use
RCG2 as a mux.
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
Signed-off-by: Robert Marko <robimarko@gmail.com>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Bjorn Andersson <andersson@kernel.org>
Link: https://lore.kernel.org/r/20220818220628.339366-1-robimarko@gmail.com