Add support for SST25LF020A.

Tested on hardware by Idwer too.

Signed-off-by: Idwer Vollering <vidwer@gmail.com>
Acked-by: Stefan Tauner <stefan.tauner@alumni.tuwien.ac.at>

git-svn-id: svn://flashrom.org/flashrom/trunk@1787 2b7e53f0-3cfb-0310-b3e9-8179ed1497e1