AP_Mount: add support for Siyi ZT6

[rmackay9]

Whenever Siyi releases a new model of camera we need to update two parts of the Siyi driver:

1. Add a new entry to the HardwareModel enum
2. Add a new entry to the hardware_lookup_table which maps the 2 byte hardware id provided by the gimbal itself to the model name displayed to the user
3. Update get_zoom_mult_max() with the maximum zoom level for the camera
4. Search for any other uses of "HardwareModel"

For reference these are the things we need to know about the camera:

1. 2 byte hardware id
2. model name
3. zoom max (e.g. 30x)
4. focal length in mm (e.g. 21, 5.15)
5. does it have a rangefinder?
6. does it have multiple lenses?

This request comes from this discussion