mvebu: Introduce kconfig symbols for SoC variants

This patch adds intermediate kconfig symbols which select their SoC
family. Boards then select them instead of the family symbol directly.

Signed-off-by: Phil Sutter <phil@nwl.cc>
Acked-by: Stefan Roese <sr@denx.de>
Reviewed-by: Tom Rini <trini@konsulko.com>