Merge remote-tracking branch 'remotes/rth-gitlab/tags/pull-nios-20220303' into staging

Rewrite nios2 interrupt handling

# gpg: Signature made Thu 03 Mar 2022 19:52:33 GMT
# gpg:                using RSA key 7A481E78868B4DB6A85A05C064DF38E8AF7E215F
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* remotes/rth-gitlab/tags/pull-nios-20220303:
  target/nios2: Rewrite interrupt handling
  target/nios2: Special case ipending in rdctl and wrctl
  target/nios2: Split mmu_write
  target/nios2: Hoist R_ZERO check in rdctl
  target/nios2: Only build mmu.c for system mode
  target/nios2: Replace MMU_LOG with tracepoints
  target/nios2: Remove mmu_read_debug

Signed-off-by: Peter Maydell <[email protected]>