clk: qcom: Add simple regmap based muxes

Add support for muxes that use regmap instead of readl/writel
directly. We don't support as many features as clk-mux.c, but
this is good enough to support getting and setting parents.
Adding a table based lookup can be added in the future if needed.

Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
Tested-by: Kenneth Westfield <kwestfie@codeaurora.org>
Signed-off-by: Michael Turquette <mturquette@linaro.org>