Support overwriting mode_bits for pb_type through bitstream setting file

[tangxifan]

Motivate of the pull request

• To address an existing issue. If so, please provide a link to the issue:
• Breaking new feature. If so, please describe details in the description part.

Describe the technical details

What is currently done? (Provide issue link if applicable)

Currently, OpenFPGA has the following limitations:

• The mode-selection bits mode_bits of any pb_type can only be defined through the OpenFPGA architecture description file. See details at link

• In practice, we need different sets of mode_bits for unused/unmapped pb_types, when performing sign-off on an FPGA fabric. During the sign-off stage, OpenFPGA architecture description file is typically forzen as it contains critical architecture-level information. Therefore, we need another file, i.e., the bitstream setting file, which can overwrite mode_bits over the architecture description file. As such, various bitstreams can be auto-generated for sign-off purpose without modifying the architecture description file.

What does this pull request change?

This PR improves in the following aspects:

• Added new syntax default_mode_bits to bitstream setting file and support default mode bits overloading
• Added associated regression tests
• Update documentation

Which part of the code base require a change

• VPR
• Tileable routing architecture generator
• OpenFPGA libraries
• FPGA-Verilog
• FPGA-Bitstream
• FPGA-SDC
• FPGA-SPICE
• Flow scripts
• Architecture library
• Cell library
• Documentation
• Regression tests
• Continous Integration (CI) scripts

Impact of the pull request

• Require a change on Quality of Results (QoR)
• Break back-compatibility. If so, please list who may be influenced.