MAINTAINERS: Fix Dialog search term for watchdog binding file

Commit 34026764 ("MAINTAINERS: da9062/61 updates to the Dialog
Semiconductor search terms") contained a typo for the watchdog binding:
da92??-wdt.txt should have read da90??-wdt.txt. This new commit will fix
the error and allows Dialog Semiconductor to follow files for PMIC
bindings correctly.

Signed-off-by: Steve Twiss <stwiss.opensource@diasemi.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>