kernel: move MTD_SPLIT_SUPPORT to generic config

CONFIG_MTD_SPLIT_SUPPORT symbol default value is 'y' and many platform
specific configs explicitly enables it, while no one platform disables
this symbol. So place it in generic config and remove from platform
specific configs.

Signed-off-by: Sergey Ryazanov <ryazanov.s.a@gmail.com>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@44612 3c298f89-4303-0410-b956-a3cf2f4a3e73