staging: nvec: Augment TODO file with GPIO work item

To make sure that this driver does not leave staging before it
is properly converted to use the new GPIO descriptor API,
augment the TODO file with this work item.

Cc: Marc Dietrich <marvin24@gmx.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>