watchdog: mt7621_wdt/rt2880_wdt: Fix compilation problem

These files need
   #include <linux/mod_devicetable.h>
to compile correctly.

Fixes: ac316725 ("headers: separate linux/mod_devicetable.h from linux/platform_device.h")
Signed-off-by: NeilBrown <neil@brown.name>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>