gpio: do not include <asm/arch/gpio.h> on TARGET_BCMNS3

As no gpio.h is defined for this architecture, to avoid
compilation failure, do not include <asm/arch/gpio.h> for
arch bcmns3.

Signed-off-by: Rayagonda Kokatanur <rayagonda.kokatanur@broadcom.com>
Reviewed-by: Simon Glass <sjg@chromium.org>