regulator: stpmic1: Simplify regulators registration

Stop using a regulator_init callback. This leads to a more simple regulator
registration code. This also permits to spuress struct stpmic1_regulator.
Also rename stpmic1_regulators_matches to stpmic1_matches.

Signed-off-by: pascal paillet <p.paillet@st.com>
Signed-off-by: Mark Brown <broonie@kernel.org>