Hello maeda,
I checked and there is documentation on this, and actually it is included in the package, you can find it here: build/tmp/work/edison-poky-linux/linux-yocto/3.10.17+gitAUTOINC+6ad20f049a_c03195ed6e-r0/linux/Documentation/devicetree/binding/gpio/gpio-poweroff.txt
Peter.