mmc: sdhci-s3c: Keep a copy of platform data and use it

The platform data is copied into driver's private data and the copy is
used for all access to the platform data. This simpifies the addition
of device tree support for the sdhci-s3c driver.

Signed-off-by: Thomas Abraham <thomas.abraham@linaro.org>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>
Signed-off-by: Chris Ball <cjb@laptop.org>