dt-bindings: hwmon: Add Apple System Management Controller hwmon schema

Apple Silicon devices integrate a vast array of sensors, monitoring
current, power, temperature, and voltage across almost every part of
the system. The sensors themselves are all connected to the System
Management Controller (SMC). The SMC firmware exposes the data
reported by these sensors via its standard FourCC-based key-value
API. The SMC is also responsible for monitoring and controlling any
fans connected to the system, exposing them in the same way.

For reasons known only to Apple, each device exposes its sensors with
an almost totally unique set of keys. This is true even for devices
which share an SoC. An M1 Mac mini, for example, will report its core
temperatures on different keys to an M1 MacBook Pro. Worse still, the
SMC does not provide a way to enumerate the available keys at runtime,
nor do the keys follow any sort of reasonable or consistent naming
rules that could be used to deduce their purpose. We must therefore
know which keys are present on any given device, and which function
they serve, ahead of time.

Add a schema so that we can describe the available sensors for a given
Apple Silicon device in the Devicetree.

Reviewed-by: Neal Gompa <neal@gompa.dev>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Signed-off-by: James Calligeros <jcalligeros99@gmail.com>
Acked-by: Guenter Roeck <linux@roeck-us.net>
Link: https://patch.msgid.link/20251215-macsmc-subdevs-v6-1-0518cb5f28ae@gmail.com
Signed-off-by: Rob Herring (Arm) <robh@kernel.org>

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