target/openrisc: add numcores and coreid support

These are used to identify the processor in SMP system.  Their
definition has been defined in verilog cores but it not yet part of the
spec but it will be soon.

The proposal for this is available:
  https://openrisc.io/proposals/core-identifier-and-number-of-cores

Reviewed-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Stafford Horne <shorne@gmail.com>

target/openrisc/sys_helper.c

@@ -233,6 +233,12 @@ target_ulong HELPER(mfspr)(CPUOpenRISCState *env,
     case TO_SPR(0, 64): /* ESR */
         return env->esr;
 
+    case TO_SPR(0, 128): /* COREID */
+        return 0;
+
+    case TO_SPR(0, 129): /* NUMCORES */
+        return 1;
+
     case TO_SPR(1, 512) ... TO_SPR(1, 512+DTLB_SIZE-1): /* DTLBW0MR 0-127 */
         idx = spr - TO_SPR(1, 512);
         return env->tlb->dtlb[0][idx].mr;