clk: keystone: Fix discarded const qualifiers

clk: keystone: Fix discarded const qualifiers

Add const qualifiers to the pointers returned from 'container_of' macro
to prevent breaking the const promise on const struct pointers from
parameters.

Once you have a mutable container structure pointer, you can change
structure fields through it, which violates the const guarantee.

Signed-off-by: Adrian Barnaś <abarnas@google.com>
Reviewed-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>