dt-bindings: power: apple,pmgr-pwrstate: Add apple,min-state prop

A few devices (DCP/DCPEXT) need to have the minimum power state for
auto-PM configured. Add a property that allows the DT to specify this
value.

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