Documentation: gpio: Add details of open-drain/source configuration

Adding details of open drain(open collector) and open source
(open emitter) configuration of the gpio so that client can
set the pin as open drain at the time of gpio request.

Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
Reviwed-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>