dt-bindings: gpio: add common consumer GPIO lines

Typical GPIO lines like enable, powerdown, reset or wakeup are not
documented as common, which leads to new variations of these (e.g.
pwdn-gpios).  Add a common schema which serves also as a documentation
for preferred naming.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>