Interrupt priority values discrepancy between major and external interrupts

[evgeniy-paltsev]

The current ACLIC specification implies that interrupt priority values for major and external interrupts are identical.

However, according to AIA spec, that's not the case:

• Priority value 0 in the iprio array is valid for major interrupts, but not for external interrupts. Moreover, the interpretation of 0 depends on the specific major interrupt.
• IPRIOLEN differs between major and external interrupts:
  • For external interrupts (APLIC), IPRIOLEN is in the range 1-8
  • For major interrupts (the iprio array), IPRIOLEN must be 6, 7, or 8.

The major and external interrupts unification approach (#606) resolves this inconsistency.