AVR32: Include <div64.h> instead of <asm/div64.h>

include/asm-avr32/div64.h was recently moved to include/div64.h, but
cpu/at32ap/interrupts.c wasn't properly updated (an earlier version of
the patch was merged perhaps?)

This patch updates cpu/at32ap/interrupts.c so that the avr32 port
compiles again.

Signed-off-by: Haavard Skinnemoen <hskinnemoen@atmel.com>

cpu/at32ap/interrupts.c

@@ -20,8 +20,8 @@
  * MA 02111-1307 USA
  */
 #include <common.h>
+#include <div64.h>
 
-#include <asm/div64.h>
 #include <asm/errno.h>
 #include <asm/io.h>
 #include <asm/processor.h>