[mie] USIE/UTIE/UEIE are writable although N is not implemented

[kkapapu]

Describe the bug

NutShell implements RV64IMAC (no N extension).

li t0, 0x111        # USIE | UTIE | UEIE
csrw mie, t0
csrr s0, mie        # DUT: 0x111

Expected behavior

Unimplemented interrupt-enable bits must be hardwired to zero (WARL / reserved). Without N, mie.USIE/UTIE/UEIE (bits 0/4/8) must read as 0.

• Expected: mie = 0
• Actual: mie = 0x111
  MaskedRegMap(Mie, mie) has no write mask.

Environment

NutShell: 041f694