pm: device: Fix if devicetree 'wakeup-source' prop isnt defined

Its possible that a dts binding doesn't inherit from base.yaml and
thus doesn't have `wakeup-source` defined.  To handle these cases
use DT_PROP_OR() which can deal with a property not existing at
all.

Fixes #37676

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>