psm: Drop bpsm%d device files. Instead support non-blocking reads on psm%d.

psm: Drop bpsm%d device files. Instead support non-blocking reads on psm%d.

The /dev/psm%d vs. /dev/bpsm%d separation doesn't serve any clear purpose
nowadays. Userland can just use fcntl(2) to switch the fd to non-blocking
or blocking mode as needed.