ARM: s3c64xx: include header to fix -Wmissing-prototypes

Include the spi-s3c64xx.h header to fix W=1 build warning:

    arch/arm/mach-s3c64xx/setup-spi.c:11:5: warning:
        no previous prototype for 's3c64xx_spi0_cfg_gpio' [-Wmissing-prototypes]
       11 | int s3c64xx_spi0_cfg_gpio(void)

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Reviewed-by: Tomasz Figa <tomasz.figa@gmail.com>
Reviewed-by: Stephen Boyd <sboyd@kernel.org>