tmc: Add support for a get_status() method

Export mcu_phase_offset and drv_status information.

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

docs/Status_Reference.md

@@ -311,6 +311,21 @@ objects:
   temperature seen by the sensor since the Klipper host software was
   last restarted.
 
+## tmc drivers
+
+The following information is available in
+[TMC stepper driver](Config_Reference.md#tmc-stepper-driver-configuration)
+objects (eg, `[tmc2208 stepper_x]`):
+- `mcu_phase_offset`: The micro-controller stepper position
+  corresponding with the driver's "zero" phase. This field may be null
+  if the phase offset is not known.
+- `phase_offset_position`: The "commanded position" corresponding to
+  the driver's "zero" phase. This field may be null if the phase
+  offset is not known.
+- `drv_status`: The results of the last driver status query. (Only
+  non-zero fields are reported.) This field will be null if the driver
+  is not enabled (and thus is not periodically queried).
+
 ## toolhead
 
 The following information is available in the `toolhead` object