clk: ti: dra7: constify clk_hw_omap_ops structure

The clk_hw_omap_ops structures are never modified, so declare this one as
const, like the others.

Done with the help of Coccinelle.

Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>