extcon: gpio: Use power efficient workqueue for debounce

The debounce timeout is generally quite long and the work not performance
critical so allow the scheduler to run the work anywhere rather than in
the normal per-CPU workqueue.

Signed-off-by: Mark Brown <broonie@linaro.org>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: Myungjoo Ham <myungjoo.ham@samsung.com>