dt-bindings: regulator: update fixed-regulator example

Fixed regulators do not have associated bus addresses and are typically
placed directly under the root node where their names must still be
unique despite not having a unit address.

Fix the malformed example node which had a unit address but no "reg"
property by dropping the unit address.

Also, try to make the example more useful by using the recommended
generic node name "regulator", but with a suffix reflecting the
regulator name in order to make it unique.

Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Mark Brown <broonie@kernel.org>