dt-bindings: arm: apple: Add apple,pmgr binding

The PMGR block in Apple Silicon SoCs is responsible for SoC power
management. There are two PMGRs in T8103, with different register
layouts but compatible registers. In order to support this as well
as future SoC generations with backwards-compatible registers, we
declare these blocks as syscons and bind to individual registers
in child nodes. Each register controls one SoC device.

The respective apple compatibles are defined in case device-specific
quirks are necessary in the future, but currently these nodes are
expected to be bound by the generic syscon driver.

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