clk: mux: Split out register accessors for reuse

We want to reuse the logic in clk-mux.c for other clock drivers
that don't use readl as register accessors. Fortunately, there
really isn't much to the mux code besides the table indirection
and quirk flags if you assume any bit shifting and masking has
been done already. Pull that logic out into reusable functions
that operate on an optional table and some flags so that other
drivers can use the same logic.

Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Georgi Djakov <georgi.djakov@linaro.org>