arm64: select ARCH_WANT_OPTIONAL_GPIOLIB

An architecture should not unconditionally enable 'GENERIC_GPIO'
without providing an implementation. In case of arm64, selecting
ARCH_WANT_OPTIONAL_GPIOLIB is the right solution, because it
lets us enable GPIOLIB when configuring the kernel, and that
implicitly turns on GENERIC_GPIO.

Reported-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>