staging: greybus: Add TODO file with GPIO work items

To make sure that these drivers do not leave staging before they
are properly converted to use the new GPIO descriptor API, and the
GPIOLIB_IRQCHIP helper library, create the TODO file with these work
items.

Cc: Johan Hovold <johan@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>