clk: fixed-rate: Document accuracy member

This member isn't documented, leading to kernel-doc warnings. Document
it.

Cc: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>
Link: https://lkml.kernel.org/r/20190830150923.259497-6-sboyd@kernel.org