Ad5754r update driver and examples

[ribdp]

Description

1. Update driver to support additional attributes defined in the firmware
2. Remove typos from the example script
3. Update example script to demonstrate use of the dac attributes

Fixes # (issue)

Type of change

Please delete options that are not relevant.

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] This change requires a documentation update

How has this been tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

• [x] Tested driver and example script with SDP-K1 and EVAL-AD5754REBZ

Test Configuration:

• Hardware: SDP-K1, EVAL-AD5754REBZ
• OS: ad5754r iio firmware application running on SDP-K1; Python 3.7 on Windows 10

Documentation

If this is a new feature or example please mention or link any documentation. All new hardware interface classes require documentation.

Checklist:

• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have signed off all commits and they contain "Signed-off by: "
• [ ] I have made corresponding changes to the documentation
• [ ] My changes generate no new warnings
• [ ] I have added tests that prove my fix is effective or that my feature works
• [ ] New and existing unit tests pass locally with my changes
• [ ] Any dependent changes have been merged and published in downstream modules

[ribdp]

Hi @tfcollins - this PR is ready for review