spi: s3c64xx: Allow build on all Samsung platforms

Replace all symbols by simple dependency PLAT_SAMSUNG.

Signed-off-by: Mateusz Krawczuk <m.krawczuk@partner.samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Mark Brown <broonie@linaro.org>