clk: fixed-rate: Convert to clk_hw based APIs

This code still uses struct clk to register clks from the probe path.
Migrate this to the clk_hw based APIs to modernize the code. Also, this
isn't a module and it can't be one because the driver is always builtin
so drop the module table.

Cc: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>
Link: https://lkml.kernel.org/r/20190830150923.259497-3-sboyd@kernel.org