regulator: Mark supply_name const and duplicate it as such

The supply_name member of struct regulator can be const as we
don't change it in the regulator core. Furthermore, when we copy
the supply name we can use kstrdup_const() here to avoid a copy
if the name is in the ro data section.

Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Mark Brown <broonie@kernel.org>