hw/a9mpcore: Fix compilation failure if physaddrs are 64 bit

Add a cast to a logging printf to avoid a compilation failure
if target_phys_addr_t is a 64 bit type. (This is better than
using TARGET_FMT_plx because we really don't need a full
16 digit hex string to print the offset into a device.)
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>
Reviewed-by: NEric Blake <eblake@redhat.com>

hw/a9mpcore.c

@@ -75,7 +75,7 @@ static void a9_scu_write(void *opaque, target_phys_addr_t offset,
         break;
     default:
         fprintf(stderr, "Invalid size %u in write to a9 scu register %x\n",
-                size, offset);
+                size, (unsigned)offset);
         return;
     }