Implement ACLIC as a subset of APLIC MMIO interface

[evgeniy-paltsev]

This PR proposes a memory-mapped ACLIC interface based on a subset of the AIA Advanced Platform-Level Interrupt Controller (APLIC).

Advantages of the proposed interface

• APLIC compatibility
  • the interface maximize compatibility with APLIC
  • ACLIC can be part of full-sized APLIC in the system
  • ACLIC can be trivially replaced by a full-sized APLIC if needed
• ACLIC interface in MMIO is cost-efficient
  • the CSR/iCSR implementations are more costly as they must provide atomic operations, which are not required for ACLIC interface
  • AIA requests simple MMIO implementation: only 32-bit accesses, no unaligned accesses
  • we don't need to implement iCSR extension, unless required by other extensions
  • system have to implement other MMIO devices anyway (e.g., mtime, mtimecmp registers for timer)
• Incoming MSI port
  • the setipnum register can be used as an incoming port for MSIs, similar to APLIC
• Software convenience
  • the interface is optimized for access by interrupt identity number (IID), reducing field position calculations
  • register access is performed with a single instruction, unlike iCSRs which require two accesses (xiselect + xireg) and require locking in OS environment

Note: if required for specific use cases, the interface can alternatively be implemented via iCSRs through a separate extension.