clk: divider: export clk_div_mask() helper

Export clk_div_mask() in clk-provider header so every clock providers
derived from the generic clock divider may share the definition instead
of redefining it.

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