hwmon: (s3c-hwmon) Disable build for S3C64xx

The s3c-hwmon driver depends on the arch/arm implementation of the core
ADC support for the chip.  Since the S3C64xx version has not yet been
merged disable building of the driver on S3C64xx for now.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Cc: Ben Dooks <ben-linux@fluff.org>
Signed-off-by: Jean Delvare <khali@linux-fr.org>