Fixed NOR extended cache zero base handling

[fdesbiens]

Summary

Fixed NOR extended cache handling when lx_nor_flash_base_address is 0.

The extended cache previously used LX_NULL as the implicit invalid cache-entry marker, which made address zero unusable as a valid logical NOR flash address. This change adds an explicit cache-entry validity flag and updates read/write/block erase cache handling accordingly.

Testing

• Added a regression-only zero-base NOR driver that treats flash addresses as logical offset tokens.
• Verified cache fill, repeated read cache hit, write cache update, and block erase invalidation at logical address zero.
• Code compiles and full regression test suite passes.