ARM: SAMSUNG: Declare struct platform_device in plat/s3c64xx-spi.h

The function declarations in plat/s3c64xx-spi.h rely on struct
platform_device but it's not declared by the header causing compiler
warnings if it is included prior to another header which does that.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>