MAINTAINERS: add diag288_wdt driver to s390 maintained files

The diag288_wdt watchdog driver is s390 specific.
Document who is responsible for this driver.
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>

MAINTAINERS

@@ -18260,6 +18260,7 @@ F:	Documentation/driver-api/s390-drivers.rst
 F:	Documentation/s390/
 F:	arch/s390/
 F:	drivers/s390/
+F:	drivers/watchdog/diag288_wdt.c
 
 S390 COMMON I/O LAYER
 M:	Vineeth Vijayan <vneethv@linux.ibm.com>