clk: socfpga: add const to _ops data structures

All the static clk_ops data structure need a const.

Signed-off-by: Dinh Nguyen <dinguyen@kernel.org>
Link: https://lkml.kernel.org/r/20200512181647.5071-3-dinguyen@kernel.org
Signed-off-by: Stephen Boyd <sboyd@kernel.org>