staging: emxx_udc: Add GPIO descriptor work to TODO

To make sure this driver does not leave staging without a proper
conversion to the GPIO descriptor API, leave a note in the TODO.

Cc: Magnus Damm <damm+renesas@opensource.se>
Cc: Simon Horman <horms+renesas@verge.net.au>
Cc: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>