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>

arch/arm/plat-samsung/include/plat/s3c64xx-spi.h

@@ -11,6 +11,8 @@
 #ifndef __S3C64XX_PLAT_SPI_H
 #define __S3C64XX_PLAT_SPI_H
 
+struct platform_device;
+
 /**
  * struct s3c64xx_spi_csinfo - ChipSelect description
  * @fb_delay: Slave specific feedback delay.