microblaze: remove ARCH_WANT_OPTIONAL_GPIOLIB

This symbols is not needed to get access to selecting the
GPIOLIB anymore: any arch can select GPIOLIB.

Cc: Michael Büsch <m@bues.ch>
Cc: Michal Simek <monstr@monstr.eu>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>