mtd: Fix Elan SC520 dependencies

Anyone working with an AMD Elan SC520 development or evaluation board
would be building a dedicated kernel for it, so we can make the
sc520cdp and netsc520 maps 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>
Cc: David Woodhouse <dwmw2@infradead.org>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>