metag: tz1090: select and instantiate gpio-tz1090

Select ARCH_WANT_OPTIONAL_GPIOLIB from SOC_TZ1090 to allow GPIOLIB and
GPIO_TZ1090 (the main gpio driver) to be enabled once it is merged, and
instantiate it from tz1090.dtsi.

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Grant Likely <grant.likely@linaro.org>