Add _PSx names to the METHOD_NAME list.

Will be used by iASL.
iASL: Add error checking for proper use of _PSx power methods.

Check to ensure the following are true:

1) _PS0 - One of these must exist: _PS1, _PS2, _PS3
2) _PS1/_PS2/_PS3: A _PS0 must exist
Merge branch 'master' of ssh://ssh.github.com/acpica/acpica