regulator: of: Skip disabled regulator nodes

If a regulator is listed in devicetree, but the node is marked as
"disabled" we should skip parsing the regulator init data and
deny consumers from interacting with the regulator. This
simplifies devicetree maintenance where we can have one dtsi file
with all regulators supported by a PMIC and then select what
regulators are used depending on the board configuration.

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