Add M355 to the GCodes list.

Marlin/Marlin_main.cpp

  * M302 - Allow cold extrudes, or set the minimum extrude S<temperature>. (Requires PREVENT_COLD_EXTRUSION)
  * M303 - PID relay autotune S<temperature> sets the target temperature. Default 150C. (Requires PIDTEMP)
  * M304 - Set bed PID parameters P I and D. (Requires PIDTEMPBED)
+ * M355 - Turn the Case Light on/off and set its brightness. (Requires CASE_LIGHT_PIN)
  * M380 - Activate solenoid on active extruder. (Requires EXT_SOLENOID)
  * M381 - Disable all solenoids. (Requires EXT_SOLENOID)
  * M400 - Finish all moves.