clk: mmp: pxa168: fix const-correctness

While working on this series of patches, checkpatch recommended that
an extra const should be added to the mux parent arrays.

Signed-off-by: Doug Brown <doug@schmorgal.com>
Link: https://lore.kernel.org/r/20220612192937.162952-6-doug@schmorgal.com


Signed-off-by: Stephen Boyd <sboyd@kernel.org>