clk: fixed-rate: Move to_clk_fixed_rate() to C file

The only user of this macro is the fixed rate basic type. Move it there
to avoid polluting provider drivers.

Cc: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>
Link: https://lkml.kernel.org/r/20190830150923.259497-5-sboyd@kernel.org