arm: exynos: Include missing CPU header in gpio.h

arch/arm/include/asm/arch/gpio.h relies on definitions from cpu.h.
Include it explicitly in gpio.h. Otherwise next build error may occur:

    In file included from ./arch/arm/include/asm/gpio.h:7,
                     from include/cros_ec.h:14,
                     from board/samsung/common/board.c:8:
        ./arch/arm/include/asm/arch/gpio.h:1357:4:
              error: 'EXYNOS4_GPIO_PART1_BASE' undeclared here
              (not in a function); did you mean 'EXYNOS4_GPIO_MAX_PORT'?
         1357 |  { EXYNOS4_GPIO_PART1_BASE, EXYNOS4_GPIO_MAX_PORT_PART_1 },
              |    ^~~~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>