watchdog: Fix Elan SC520 dependencies

Anyone using a system based on an AMD Elan SC520 processor would be
building a dedicated kernel for it, so we can make the sc520_wdt
driver depend on MELAN. SC520_CPUFREQ already depends on MELAN so it
makes things more consistent. It also makes kernel configuration for
every other x86 user easier.

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>