watchdog: OMAP: fixup for ARM multiplatform support

Recent changes to the omap_wdt.c removed the dependencies to
the core omap code, but forgot to remove mach/hardware.h.

We cannot include any plat headers with multiplatform
support enabled.

cc: Wim Van Sebroeck <wim@iguana.be>
cc: linux-watchdog@vger.kernel.org
Signed-off-by: Tony Lindgren <tony@atomide.com>