fix: invalidate dcache when PMP permissions change

[KnightGOKU]

Fixes #58

Summary

PMP permission changes can leave stale data in the D-cache and in the MCDATA registers, allowing cache-index reads to expose data that is no longer accessible through normal loads.

Changes

• Trigger a D-cache invalidation on every PMP CSR update.
• Clear the MCDATA0/MCDATA1 latches when PMP permissions change.
• Allow PMP-triggered invalidation requests to be queued while the ICC state machine is busy.

Validation

Verilator block-level tests pass for:

• PMP-triggered D-cache invalidation
• MCDATA clearing after a PMP update
• Invalidation requests arriving while ICC is busy

The patch is limited to the cache diagnostic and PMP-update paths. The invalidation is conservative and may flush the D-cache after each PMP CSR write.