Add PVR register to mfspr

PVR stores the processor/motherboard revision, the latest I could find seems to be 0x710800 which is probably a Corona.
XShell tries to retrieve this during launch to print it on the console (search "XSHELL: CPUPVR" on pastebin for some examples)
Xenia crashes because it's unimplemented, so I just made it return 0x710800. It could be worth moving this to be a constant somewhere though.

IIRC hypervisor/kernel uses this for some things too, but since we don't emulate those it's not much of a problem, but there could still be other XEXs which use it for something.

src/xenia/cpu/ppc/ppc_emit_control.cc

@@ -616,6 +616,10 @@ int InstrEmit_mfspr(PPCHIRBuilder& f, const InstrData& i) {
       // TBU
       v = f.Shr(f.LoadClock(), 32);
       break;
+    case 287:
+      // PVR
+      v = f.LoadConstantUint64(0x710800);
+      break;
     default:
       XEINSTRNOTIMPLEMENTED();
       return 1;