Add M25P20-old.

This older (ST-branded) revision of M25P20 chip does not support RDID and
hence was not detected correctly. This patch adds a workaround similar
to M25P40-old.

Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Acked-by: Stefan Tauner <stefan.tauner@student.tuwien.ac.at>

git-svn-id: svn://coreboot.org/flashrom/trunk@1652 2b7e53f0-3cfb-0310-b3e9-8179ed1497e1