Add VIA VT6421A LPC programmer driver.

Due to the mysterious address handling of this chip the user can specify
a base address with the offset parameter, e.g.:
flashrom -p atavia:offset=0xFFF00000

Thanks to Idwer Vollering for his iterative testing of this code, as well as to
Martijn Bastiaan who did the last tests before merging.

Signed-off-by: Jonathan Kollasch <jakllsch@kollasch.net>
Signed-off-by: Stefan Tauner <stefan.tauner@alumni.tuwien.ac.at>
Acked-by: Stefan Tauner <stefan.tauner@alumni.tuwien.ac.at>

git-svn-id: svn://flashrom.org/flashrom/trunk@1809 2b7e53f0-3cfb-0310-b3e9-8179ed1497e1