ad7779: apply a sync pulse after configuration

[jsarzy]

Description

Use a START pulse to generate an internal SYNC_IN pulse after each configuration. Additionaly, I removed the START pulse and long sleeps after device reboot. I have tested this on imx6ull and imxrt based architectures and it seems to work well. Hang-ups do not occur.

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 change)

How Has This Been Tested?

• Already covered by automatic testing.
• New test added: (add PR link here).
• Tested by hand on: imx6ull, imxrt106x.

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing linter checks and tests passed.
• My changes generate no new compilation warnings for any of the targets.

Special treatment

• This PR needs additional PRs to work (list the PRs, preferably in merge-order).
• I will merge this PR by myself when appropriate.

[jsarzy]

Perhaps a less error-prone and more readable approach would be to use a struct with fields instead the array with magic indices.