sony-laptop: add usb charge function

Allows to specify if the USB socket should charge attached devices while
the laptop is suspended to ram.

Signed-off-by: Javier Achirica <jachirica@gmail.com>
Signed-off-by: Mattia Dongili <malattia@linux.it>
Signed-off-by: Matthew Garrett <matthew.garrett@nebula.com>