acpi: Don't add an empty SSDT.

It's harmless but useless.

Change-Id: Iaaa5f6933d120a2071b2e32e62e36e63afa96be3
Signed-off-by: Vladimir Serbinenko <phcoder@gmail.com>
Reviewed-on: http://review.coreboot.org/7043
Reviewed-by: Aaron Durbin <adurbin@gmail.com>
Tested-by: build bot (Jenkins)