soc: apple: apple-pmgr-pwrstate: Add auto-PM min level support

This is seemingly required for DCP/DCPEXT, without which they refuse to
boot properly. They need to be set to minimum state 4 (clock gated).

Reviewed-by: Sven Peter <sven@svenpeter.dev>
Signed-off-by: Hector Martin <marcan@marcan.st>