regulator: dt-bindings: add s2mpg11-pmic regulators

The S2MPG11 PMIC is a Power Management IC for mobile applications with
buck converters, various LDOs, power meters, NTC thermistor inputs, and
additional GPIO interfaces. It typically complements an S2MPG10 PMIC in
a main/sub configuration as the sub-PMIC.

S2MPG11 has 12 buck, 1 buck-boost, and 15 LDO rails. Several of these
can either be controlled via software (register writes) or via external
signals, in particular by:
* one out of several input pins connected to a main processor's:
* GPIO pins
* other pins that are e.g. firmware- or power-domain-controlled
without explicit driver intervention
* a combination of input pins and register writes.

Control via input pins allows PMIC rails to be controlled by firmware,
e.g. during standby/suspend, or as part of power domain handling where
otherwise that would not be possible. Additionally toggling a pin is
faster than register writes, and it also allows the PMIC to ensure that
any necessary timing requirements between rails are respected
automatically if multiple rails are to be enabled or disabled quasi
simultaneously.

While external control via input pins appears to exist on other
versions of this PMIC, there is more flexibility in this version, in
particular there is a selection of input pins to choose from for each
rail (which must therefore be configured accordingly if in use),
whereas other versions don't have this flexibility.

Add documentation related to the regulator (buck & ldo) parts like
devicetree definitions, regulator naming patterns, and additional
properties.

Since S2MPG11 is typically used as the sub-PMIC together with an
S2MPG10 as the main-PMIC, the datasheet and the binding both suffix the
rails with an 's'.

Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: André Draszik <andre.draszik@linaro.org>
Link: https://patch.msgid.link/20260122-s2mpg1x-regulators-v7-3-3b1f9831fffd@linaro.org
Signed-off-by: Mark Brown <broonie@kernel.org>