bump spike for pmp registers (#260)

srivatsa611y · aswaterman · commit d4f3490ed2bb · 2018-10-18T15:51:51.000-07:00
diff --git a/riscv-isa-sim b/riscv-isa-sim
@@ -1 +1 @@
-Subproject commit 6fecdb16d72b71734b35f494023f5edc8804327c
+Subproject commit f54ff67b56592a28ce5b5ed729ceb6ad22f7cb8d
