MAINTAINERS: Add myself to watchdog collaborators.

Currently there is no maintainer and active collaborator in watchdog
subsystem area. I hope this helps to encourage Zephyr developments.

Signed-off-by: Katsuhiro Suzuki <katsuhiro@katsuster.net>

Author: katsuster

MAINTAINERS.yml

@@ -697,6 +697,8 @@ Documentation:
 
 "Drivers: Watchdog":
     status: orphaned
+    collaborators:
+        - katsuster
     files:
         - doc/reference/peripherals/watchdog.rst
         - drivers/watchdog/