hexagon: update TODO list

The symbol that will be selected when GPIO is implemented
for Hexagon will be GPIOLIB, we have removed
ARCH_REQUIRE_GPIOLIB and ARCH_WANT_OPTIONAL_GPIOLIB.

Cc: Michael Büsch <m@bues.ch>
Cc: Richard Kuo <rkuo@codeaurora.org>
Cc: linux-hexagon@vger.kernel.org
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>