ARM64: add GPIO keys to the defconfig

The Juno board, and likely many other boards, likes to use simple
GPIO keys for input events. Enabled this in the default
ARM64 defconfig.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Shannon Zhao <shannon.zhao@linaro.org>
Signed-off-by: Kevin Hilman <khilman@linaro.org>