hw/imx_avic.c: Avoid format error when target_phys_addr_t is 64 bits

Add a missing cast to avoid gcc complaining about format string
errors when printing an expression based on a target_phys_addr_t.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Peter Chubb <peter.chubb@nicta.com.au>

hw/imx_avic.c

@@ -267,7 +267,7 @@ static void imx_avic_write(void *opaque, target_phys_addr_t offset,
     /* Vector Registers not yet supported */
     if (offset >= 0x100 && offset <= 0x2fc) {
         IPRINTF("imx_avic_write to vector register %d ignored\n",
-                (offset - 0x100) >> 2);
+                (unsigned int)((offset - 0x100) >> 2));
         return;
     }