regulator: rohm-regulator: Support SNVS HW state.

The ROHM BD71815 supports setting voltage levels/regulator status
for HW-states "RUN", "SUSPEND", "LPSR" and "SNVS". Add DT parsing
helper also for SNVS state.

Signed-off-by: Matti Vaittinen <matti.vaittinen@fi.rohmeurope.com>
Acked-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>