gpio: add function to export gpios
gpio: add function to export gpios

Many platforms don't have gpio signals available to userspace
in gpio class. This function adds support to export gpios.

Signed-off-by: Sanjay Singh Rawat <da0109ea6cf60988b7173c09a33cf15cf06ce89c@linaro.org>

yinquan529/platform-external-powerdebug,yinquan529/platform-external-powerdebug,gromaudio/android_external_powerdebug,gromaudio/android_external_powerdebug