Add support for softkeys 9-16 to SQ-Rack

SQ MixPad support softkeys 9-16 (which are present on the SQ6 and SQ7, but not on the SQ5 and SQ-Rack). But the SQ-Rack doesn’t allow for configuring them, nor are they available in SQ Control. This seems like an oversight, so I’m asking that the softkeys 9-16 be available for configuration on the SQ-Rack and in SQ Control even though they don’t exist as hardware buttons.