gpio: allow setting ARCH_NR_GPIOS from Kconfig

The ARM version of asm/gpio.h basically just contains the same definitions
as the gpiolib version, with the exception of ARCH_NR_GPIOS.

This adds the option for overriding the constant through Kconfig to
the architecture-independent header, so we can remove the ARM specific
file later.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>