Add support for the optional power-supply.

Note: A "dummy regulator" is returned by devm_regulator_get()
if the optional regulator is not present in the device tree,
simplifying the source code when enabling/disabling the regulator.
Signed-off-by: Philippe Cornu <philippe.cornu@st.com>
Reviewed-by: Yannick Fertré <yannick.fertre@st.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180205094532.23547-3-philippe.cornu@st.com