pine64-pinebookpro: add service for setting low power state on boot

This adds a very basic openrc runscript that sets the power state on /dev/nvme0 to level 2, which seems(?) to be the lowest operational power state based on some cursory internet searching (and querying the nvme drives I have, Samsung EVO and Pro)

The drive path is hard coded, but could be made configurable later on. I basically wanted to get something in pmaports quickly, since this power stuff is a major source of instability for me and probably others too who add nvme drives to this device.