dt-bindings: power: Add apple,pmgr-pwrstate binding

This syscon child node represents a single SoC device controlled by the
PMGR block. This layout allows us to declare all device power state
controls (power/clock gating and reset) in the device tree, including
dependencies, instead of hardcoding it into the driver. The register
layout is uniform.

Each pmgr-pwrstate node provides genpd and reset features, to be
consumed by downstream device nodes.

Future SoCs are expected to use backwards compatible registers, and the
"apple,pmgr-pwrstate" represents any such interfaces (possibly with
additional features gated by the more specific compatible), allowing
them to be bound without driver updates. If a backwards incompatible
change is introduced in future SoCs, it will require a new compatible,
such as "apple,pmgr-pwrstate-v2".

Reviewed-by: Mark Kettenis <kettenis@openbsd.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Hector Martin <marcan@marcan.st>