mmc: sdhci-s3c: Fix operation on non-single image Samsung platforms

Commit cc014f3 (mmc: sdhci-s3c: remove platform dependencies) changed
the platform dependency in Kconfig to depend on SAMSUNG_SINGLE, meaning
the driver was available only on single zImage Samsung platforms.  This
breaks platforms that haven't been converted such as S3C64xx.  Revert
that part of the change to fix.

[arnd: CONFIG_SAMSUNG_SINGLE was actually introduced only for
non-multiplatform configurations, but was subsequently removed,
which means the driver was now broken both for single- and multi-
platform. The dependency was there by accident anyway, so reverting
this is still the correct solution]

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>