SPEAr: Update defconfigs

This patch updates following configuration:
- Update MTD specific configurations
- Enables JFFS2 FS support
- Updates 6xx defconfig, with recent changes done by Arnd in 6xx Kconfig

Signed-off-by: Viresh Kumar <viresh.kumar@st.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Stefan Roese <sr@denx.de>