dt-bindings: bq25980: Fixup the example

Use the ti,watchdog-timeout-ms property instead of the unsupported
ti,watchdog-timer property to make the example validate correctly.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://lore.kernel.org/r/20211206152905.226239-1-thierry.reding@gmail.com
Signed-off-by: Rob Herring <robh@kernel.org>