tmc: Remove virtual_enable support and replace with automatic support

Automatically detect if the stepper has a dedicated enable line.  If
it does not, then automatically enable virtual enable support.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

config/generic-duet2.cfg

@@ -6,7 +6,7 @@
 [stepper_x]
 step_pin: PD6
 dir_pin: PD11
-enable_pin: !PC6, tmc2660_stepper_x:virtual_enable
+enable_pin: !PC6
 step_distance: .0125
 endstop_pin: ^PC14
 position_endstop: 0
@@ -22,7 +22,7 @@ sense_resistor: 0.051
 [stepper_y]
 step_pin: PD7
 dir_pin: !PD12
-enable_pin: !PC6, tmc2660_stepper_y:virtual_enable
+enable_pin: !PC6
 step_distance: .0125
 endstop_pin: ^PA2
 position_endstop: 0
@@ -38,7 +38,7 @@ sense_resistor: 0.051
 [stepper_z]
 step_pin: PD8
 dir_pin: PD13
-enable_pin: !PC6, tmc2660_stepper_z:virtual_enable
+enable_pin: !PC6
 step_distance: .0025
 endstop_pin: ^PD29
 #endstop_pin: PD10  # E0 endstop
@@ -56,7 +56,7 @@ sense_resistor: 0.051
 [extruder]
 step_pin: PD5
 dir_pin: PA1
-enable_pin: !PC6, tmc2660_extruder:virtual_enable
+enable_pin: !PC6
 step_distance: .002
 nozzle_diameter: 0.400
 filament_diameter: 1.750
@@ -80,7 +80,7 @@ sense_resistor: 0.051
 #[extruder1]
 #step_pin: PD4
 #dir_pin: PD9
-#enable_pin: !PC6, tmc2660_extruder1:virtual_enable
+#enable_pin: !PC6
 #heater_pin: !PA16
 #sensor_pin: PC12
 #...