PPC: Fix L2CR write accesses

Commit 2345f1c01 was supposed to render L2CR writes into noops. Instead,
it made them illegal instruction traps which apparently didn't confuse
XNU, but can easily confuse other OSs.

Fix it up by actually doing nothing when we write to L2CR.

Reported-by: Julio Guerra <guerr@julio.in>
Signed-off-by: Alexander Graf <agraf@suse.de>
Tested-by: Julio Guerra <guerr@julio.in>
