More 8 extruder defines

Marlin/Configuration_adv.h

     #define E5_MICROSTEPS       16
   #endif
 
+  #if AXIS_DRIVER_TYPE_E6(TMC26X)
+    #define E6_MAX_CURRENT    1000
+    #define E6_SENSE_RESISTOR   91
+    #define E6_MICROSTEPS       16
+  #endif
+
+  #if AXIS_DRIVER_TYPE_E7(TMC26X)
+    #define E7_MAX_CURRENT    1000
+    #define E7_SENSE_RESISTOR   91
+    #define E7_MICROSTEPS       16
+  #endif
+
 #endif // TMC26X
 
 // @section tmc_smart

Marlin/src/inc/SanityCheck.h

   INVALID_TMC_ADDRESS(E4);
 #elif AXIS_DRIVER_TYPE_E5(TMC2209)
   INVALID_TMC_ADDRESS(E5);
+#elif AXIS_DRIVER_TYPE_E6(TMC2209)
+  INVALID_TMC_ADDRESS(E6);
+#elif AXIS_DRIVER_TYPE_E7(TMC2209)
+  INVALID_TMC_ADDRESS(E7);
 #endif
 #undef INVALID_TMC_ADDRESS
 

Marlin/src/module/stepper/TMC26X.cpp

 #if AXIS_DRIVER_TYPE_E5(TMC26X)
   _TMC26X_DEFINE(E5);
 #endif
+#if AXIS_DRIVER_TYPE_E6(TMC26X)
+  _TMC26X_DEFINE(E6);
+#endif
+#if AXIS_DRIVER_TYPE_E7(TMC26X)
+  _TMC26X_DEFINE(E7);
+#endif
 
 #define _TMC26X_INIT(A) do{ \
   stepper##A.setMicrosteps(A##_MICROSTEPS); \
   #if AXIS_DRIVER_TYPE_E5(TMC26X)
     _TMC26X_INIT(E5);
   #endif
+  #if AXIS_DRIVER_TYPE_E6(TMC26X)
+    _TMC26X_INIT(E6);
+  #endif
+  #if AXIS_DRIVER_TYPE_E7(TMC26X)
+    _TMC26X_INIT(E7);
+  #endif
 }
 
 #endif // TMC26X

Marlin/src/module/temperature.h

 typedef enum : int8_t {
   INDEX_NONE = -5,
   H_PROBE, H_REDUNDANT, H_CHAMBER, H_BED,
-  H_E0, H_E1, H_E2, H_E3, H_E4, H_E5
+  H_E0, H_E1, H_E2, H_E3, H_E4, H_E5, H_E6, H_E7
 } heater_ind_t;
 
 // PID storage