MAINTAINERS: da9062/61 updates to the Dialog Semiconductor search terms

Additions to search terms for files supported by Dialog Semiconductor.
This update will allow Dialog support to follow files for device tree
bindings (onkey, thermal and watchdog) and source code for chip thermal
monitoring drivers.

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