N-extension support

Created by: shetalani

The privileged specification "The RISC-V Instruction Set Manual, Volume II: Privileged Architecture" states that: "If user-level interrupts are omitted, the UIE and UPIE bits are hardwired to zero.", Which is not the case when the parameter PULP_SECURE is set, as the current implemented misa doesn't have the N extension. Is there a reason for this?