[otbn] Wrap bn.movr increments modulo 32

This is what the RTL did already. Make the spec match the equivalent
documentation for bn.lid and update the ISS to match.

Signed-off-by: Rupert Swarbrick <rswarbrick@lowrisc.org>
diff --git a/hw/ip/otbn/dv/otbnsim/sim/insn.py b/hw/ip/otbn/dv/otbnsim/sim/insn.py
index 306cb7a..a125c7a 100644
--- a/hw/ip/otbn/dv/otbnsim/sim/insn.py
+++ b/hw/ip/otbn/dv/otbnsim/sim/insn.py
@@ -886,11 +886,11 @@
         state.wdrs.get_reg(wrd).write_unsigned(value)
 
         if self.grd_inc:
-            new_grd_val = (grd_val + 1) & ((1 << 32) - 1)
+            new_grd_val = (grd_val + 1) & 0x1f
             state.gprs.get_reg(self.grd).write_unsigned(new_grd_val)
 
         if self.grs_inc:
-            new_grs_val = (grs_val + 1) & ((1 << 32) - 1)
+            new_grs_val = (grs_val + 1) & 0x1f
             state.gprs.get_reg(self.grs).write_unsigned(new_grs_val)