cpufreq: s3c64xx: Rename index to driver_data

The index field of cpufreq_frequency_table has been renamed to
driver_data by commit 50701588 (cpufreq: rename index as driver_data
in cpufreq_frequency_table).

This patch updates the s3c64xx driver to match.

Signed-off-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Cc: 3.11+ <stable@vger.kernel.org> # 3.11+
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>