Beckhoff: fill in DESC with description of axis

[rerpha]

As a developer I would like to reduce the amount of manual steps taken when setting a Beckhoff on an instrument. One of these steps at the moment is filling in the .DESC field on the motor PV with a description of each Axis.

Acceptance Criteria

• Discussion is had with Motion team to talk about how to add this information to the ST struct/global struct
• This mechanism is utilised in the TWINCAT IOC, and when setting up the axes the DESC field is filled in
• IOC tests are written for this.

Extra Information

Currently this information does not exist in the tpy file, maybe a good place to put it would be the globals struct (where things live that don't change dynamically, such as nAXIS_NUM which is the number of axes on a particular controller) - this is slightly problematic as we haven't got access to this information before an IOCInit therefore can't get its value when setting up the axes.

[DominicOram]

It probably makes more sense to be in the Axis struct as it's per axis rather than global.

[rerpha]

done in #7979