spi: s3c64xx: Always select S3C64XX_PL080 when ARCH_S3C64XX is enabled

The legacy S3C64xx DMA driver has been removed, DMA support on
S3C64xx is provided only by the generic PL08x driver.

This patch modifies the Kconfig entry of spi-s3c64xx driver, which
relies on availability of DMA, to always select the S3C64XX_PL080
symbol.

Signed-off-by: Tomasz Figa <tomasz.figa@gmail.com>
Signed-off-by: Mark Brown <broonie@linaro.org>