aarch64 structures: constrain irq_w field width

The rest of the code uses 9-bit irq width. Keep in sync here to ease
verification.

Signed-off-by: Gerwin Klein <gerwin.klein@proofcraft.systems>

include/arch/arm/arch/64/mode/object/structures.bf

@@ -203,7 +203,8 @@ block VCPUFault {
 }
 
 block VPPIEvent {
-    field irq_w     64
+    padding         55
+    field irq_w      9
     padding         32
     padding         28
     field seL4_FaultType  4