spi: Add spi-s3c64xx driver dependency on ARCH_EXYNOS4

The spi-s3c64xx driver is also used on Exynos4 so update the dependency
to enable build on those platforms.

Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
[grant.likely: relax depends to ARCH_EXYNOS instead of ARCH_EXYNOS4]
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>