ARM: S3C64XX: remove obsolete Makefile line

Commit a07613a54d70 ("Merge branch 'samsung/dt' into samsung/cleanup")
added this line to arch/arm/mach-s3c64xx/Makefile:
    obj-$(CONFIG_S3C64XX_DEV_SPI)	+= dev-spi.o

But at that time CONFIG_S3C64XX_DEV_SPI wasn't a valid Kconfig macro
anymore and arch/arm/mach-s3c64xx/dev-spi.c was already removed. So we
can remove this line.

Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>