mmc: sdhci-s3c: remove platform dependencies

plat/regs-sdhci.h is not used anywhere but in the sdhci-s3c
driver, so it can become a local file there and all other
inclusions removed.

plat/sdhci.h is used only to define the platform devices,
and with the exception of the platform_data structure not
needed by the driver, so we can split out the platform_data
definition instead and leave the rest to platform code.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Chris Ball <cjb@laptop.org>