⚠️ rename SLINK data interface registers

[stnolting]

• ⚠️ rename SLINK data interface registers: only two registers are used now: DATA and DATA_LAST; reading any of these registers will return data from the RX FIFO; writing to any of these registers will write to the TX FIFO; writing DATA_LAST will also set the "last" identifier
• minor CPU control unit optimizations (area)
• update LiteX wrapper file header
• cleanup some CFU comments and code (for better understanding)