clk: mux: add helper function for index/value translation

Add helper functions for the translation between parent index and
register value in the generic multiplexer function. The purpose of
this change is avoid duplicating the code in other clock providers,
using the same generic logic.

Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: Michael Turquette <mturquette@baylibre.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>