staging: fbtft: Add TODO file with GPIO work item

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

Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Noralf Tronnes <notro@tronnes.org>
Signed-off-by: NLinus Walleij <linus.walleij@linaro.org>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/fbtft/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, ACPI or board files, board files should
+  use <linux/gpio/machine.h>