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: NLinus Walleij <linus.walleij@linaro.org>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/greybus/TODO

+* Convert all uses of the old GPIO API from <linux/gpio.h> to the
+  GPIO descriptor API in <linux/gpio/consumer.h> and look up GPIO
+  lines from device tree or ACPI.
+* Convert the GPIO driver to use the GPIO irqchip library
+  GPIOLIB_IRQCHIP instead of reimplementing the same.