SAM9 watchdog: update for moved headers

The architecture header files were recently moved from
include/asm-arm/mach-at91/ to arch/arm/mach-at91/include/mach/.  The SAM9
watchdog driver still includes a header from the old location.

Signed-off-by: Andrew Victor <linux@maxim.org.za>
Cc: Wim Van Sebroeck <wim@iguana.be>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>