acpigen: Correctly handle root scope

Change-Id: I9b3c9109b01e348259e64e93a4397212216ab152
Signed-off-by: Vladimir Serbinenko <phcoder@gmail.com>
Reviewed-on: http://review.coreboot.org/6799
Tested-by: build bot (Jenkins)
Reviewed-by: Aaron Durbin <adurbin@google.com>