clk: honor CLK_MUX_ROUND_CLOSEST in generic clk mux

CLK_MUX_ROUND_CLOSEST is part of the clk_mux documentation but clk_mux
directly calls __clk_mux_determine_rate(), which overrides the flag.
As result, if clk_mux is instantiated with CLK_MUX_ROUND_CLOSEST, the
flag will be ignored and the clock rounded down.

To solve this, this patch expose clk_mux_determine_rate_flags() in the
clk-provider API and uses it in the determine_rate() callback of clk_mux.

Fixes: 15a02c1f ("clk: Add __clk_mux_determine_rate_closest")
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>