idma: Streamline TCDM connection, enable multi-channel operation

[thommythomaso]

Modify the iDMA integration into the Snitch cluster.

This includes:

• The wide port from the SoC directly connects to the TCDM subsystem, allowing external data access while DMA transfers run
• The wide DMA crossbar has been simplified
• The DMA uses the OBI protocol to connect to the TCDM subsystem
• Arbitration between the channels, the superbanks, and the SoC port happens now in TCDM, increasing flexibility and throughput. With minimal changes, it is possible to have multiple DMA cores in the cluster.

Before merging:

• Update custom_instructions.md
• Fix failing tests.
• Fix linting issues.
• Check for (perfomrance) regressions
• Merge the corresponding branch in the iDMA repo into main #88
• Merge the corresponding branch in the riscv-opcodes repo into main