hw/intc: add implementation of GICD_IIDR to Arm GIC

a66a24585f (hw/intc/arm_gic: Implement read of GICC_IIDR) implemented
this for the CPU interface register. The fact we don't implement it
shows up when running Xen with -d guest_error which is definitely
wrong because the guest is perfectly entitled to read it.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

hw/intc/arm_gic.c

@@ -973,8 +973,18 @@ static uint8_t gic_dist_readb(void *opaque, hwaddr offset, MemTxAttrs attrs)
             /* GICD_TYPER byte 1 */
             return (s->security_extn << 2);
         }
-        if (offset < 0x08)
+        if (offset == 8) {
+            /* GICD_IIDR byte 0 */
+            return 0x3b; /* Arm JEP106 identity */
+        }
+        if (offset == 9) {
+            /* GICD_IIDR byte 1 */
+            return 0x04; /* Arm JEP106 identity */
+        }
+        if (offset < 0x0c) {
+            /* All other bytes in this range are RAZ */
             return 0;
+        }
         if (offset >= 0x80) {
             /* Interrupt Group Registers: these RAZ/WI if this is an NS
              * access to a GIC with the security extensions, or if the GIC