spi: s3c64xx: add support for exynos7 SPI controller

Exynos7 SPI controller supports only the auto Selection of
CS toggle mode and Exynos7 SoC includes six SPI controllers.
Add support for these changes in Exynos7 SPI controller driver.

Signed-off-by: Padmavathi Venna <padma.v@samsung.com>
Signed-off-by: Mark Brown <broonie@kernel.org>