ARM: s3c24xx: allow selecting S3C2440_XTAL_16934400 for s3c2442

The S3C2440_XTAL_16934400 and S3C2440_XTAL_12000000 symbols are used
for both s3c2442 and s3c2440, but Kconfig only allows it to be selected
if CPU_S3C2440 is enabled, which can lead to a warning otherwise:

warning: (MACH_RX1950) selects S3C2440_XTAL_16934400 which has unmet direct dependencies (ARCH_S3C24XX && CPU_S3C2440)

This changes the dependencies to make it possible also for CPU_S3C2442-only
configurations.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>