Perform power down sequence from port expander trigger

Description

Add the ramping down of power procedure from trigger input (on interrupt).

How Has This Been Tested?

This has been tested by triggering the inputs and checking that the procedure starts.
Further triggering or changes on inputs while powering down also do not affect execution.

Relevant Documents (optional)

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)
• Docs (changes to the documentation)
• Style (formatting changes only, no code changes)
• Refactor (refactoring code with no functionality changes)
• Test (additional or refactored tests)
• Chore (non-code changes, e.g. comments, readme, grammar, etc...)

Checklist:

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