clk: mmp2: Constify some strings

All the parent clock names for the muxes are constant. Add const.

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Link: https://lkml.kernel.org/r/20200309194254.29009-3-lkundrak@v3.sk


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