clk: fixed: Add comment to clk_fixed_set_rate

Currently it is not made explicit why clk_fixed_set_rate() can ignore
its arguments and unconditionally return success. Add a comment
to explain this.

We also mark the clk_ops table const since it should never be
modified at runtime.

Suggested-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Daniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>