spi: s3c64xx: Do not require legacy DMA API in case of S3C64XX

With support for amba-pl08x driver, on S3C64xx the generic DMA engine
API can be used instead of the private s3c-dma interface.

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