Documentation: add a driver API doc for the power sequencing subsystem

Describe what the subsystem does, how the consumers and providers work
and add API reference generated from kerneldocs.

Acked-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20240821100818.13763-1-brgl@bgdev.pl


Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>