Extend support for AD40xx with variable sample frequency

[pamolloy]

An implementation already exists here, but we need to make the sample_frequency attribute writeable.

Existing code

adi/ad4020.py
doc/source/devices/adi.ad4020.rst
examples/ad4020_example.py
test/emu/devices/ad4020.xml
test/test_ad4020.py

Hardware

• [ ] MX1002 → EVAL-AD4000FMCZ → ZedBoard
• [ ] MX1002 → EVAL-AD4001FMCZ → ZedBoard
• [ ] MX1002 → EVAL-AD4002FMCZ → ZedBoard
• [ ] MX1002 → EVAL-AD4003FMCZ → ZedBoard
• [ ] MX1002 → EVAL-AD4020FMCZ → ZedBoard
• [ ] MX1002 → EVAL-ADAQ4001PMDZ → EVAL-PMD-IB1Z → ZedBoard
• [ ] MX1002 → EVAL-ADAQ4003PMDZ → EVAL-PMD-IB1Z → ZedBoard

Tasks

• [x] If not already setup, setup hardware for testing

Dependencies

• analogdevicesinc/hdl/issues/1152
• analogdevicesinc/precision-converters-firmware/issues/4
• analogdevicesinc/linux/issues/2239