soc: apple: apple-pmgr-pwrstate: Do not build as a module

This doesn't make any sense as a module since it is a critical device,
and it turns out of_phandle_iterator_args was not exported so the module
version doesn't build anyway.

Fixes: 6df9d38f ("soc: apple: Add driver for Apple PMGR power state controls")
Reported-by: kernel test robot <lkp@intel.com>
Reviewed-by: Sven Peter <sven@svenpeter.dev>
Signed-off-by: Hector Martin <marcan@marcan.st>