clk: versatile: make clk_ops const

Make this const as it is only stored in the const field of a
clk_init_data structure.

Signed-off-by: Bhumika Goyal <bhumirks@gmail.com>
Acked-by: Liviu Dudau <Liviu.Dudau@arm.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>