add irig_minus_sys in hwp_encoder agent

[ykyohei]

Add new data feed of irig time minus system time

Description

Add new data feed of irig time minus system time to hwp encoder agent

Motivation and Context

This is to help monitoring the PTP timing state.
With this value we can easily notice that we have synchronization loss either in NTP or PTP.

How Has This Been Tested?

This has been tested by daq-dev using hwp-bbb-a2
The minus 1 second difference is reproduced.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.

[BrianJKoopman]

Thanks for adding this.