intc/xilinx_intc: Inhibit write to ISR when HIE

When the Hardware Interrupt Enable (HIE) bit is set, software cannot
change ISR. Add write guard accordingly.

Signed-off-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Signed-off-by: Edgar E. Iglesias <edgar.iglesias@gmail.com>

hw/intc/xilinx_intc.c

@@ -116,6 +116,11 @@ pic_write(void *opaque, hwaddr addr,
         case R_CIE:
             p->regs[R_IER] &= ~value; /* Atomic clear ie.  */
             break;
+        case R_ISR:
+            if ((p->regs[R_MER] & 2)) {
+                break;
+            }
+            /* fallthrough */
         default:
             if (addr < ARRAY_SIZE(p->regs))
                 p->regs[addr] = value;