clk: qcom: Add support for root clock generators (RCGs)

Add support for the root clock generators on Qualcomm devices.
RCGs are highly customizable mux/divider/counter clocks that can
be used to generate almost any rate desired given some input
source that is faster than the desired rate.

Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Mike Turquette <mturquette@linaro.org>