mtd: nand: s3c2410: Register cpufreq notifier only on S3C24xx

The driver registered for CPU frequency transitions to recalculate its
clock when ARM clock frequency changes (ratio between frequencies of
ARM's parent clock (fclk) and clock for peripherals remains fixed).

This is needed only on S3C24xx platform when cpufreq driver is enabled
so limit the ifdef to respective cpufreq Kconfig.

Suggested-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>