linux-phytec-fsl

gpio: add a real time compliance checklist

Add some information about real time compliance to the driver document.
Inspired by Grygorii Strashko's real time compliance patches.

Cc: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>