Ensure the ADC device is touched only if S3C_DEV_ADC
is selected.

Fixes:
arch/arm/mach-s5pv210/built-in.o: In function `s5pv210_map_io':
/home/kgene/bjdooks/arch/arm/mach-s5pv210/cpu.c:92: undefined reference to `s3c_device_adc'
Signed-off-by: Ben Dooks <ben-linux@fluff.org>