voice_ctrl: fix mixmode mux

Use the unflattened mixmode signal.
Output zero when nco is disabled, instead of using the first
wavetable entry.
wb_slave: remove hardcoded signal width
wb_slave: fix missing index when flattening envelope_gate
voice_mixer: make sure second mul operand is unsigned
add gitignores for sw
add initial sw support

basically just drivers to configure the ssm2603 over i2c and
some helper functions into accessing sublime, far from complete
and contains a lot of debug printouts.