Fix ROM decoding on VIA VT82C596 and VT82C686A/B.

These support an additional bit which we did not turn on yet.
Without this patch they decode up to 512 kB, with this up to 1 MB.

Disentangle the enables of unrelated but mostly compatible chipsets
too, add some more debug output and set the max_rom_decode limits.
Also, make warnings really only warnings.

Signed-off-by: Stefan Tauner <stefan.tauner@student.tuwien.ac.at>
Acked-by: Stefan Tauner <stefan.tauner@student.tuwien.ac.at>

git-svn-id: svn://flashrom.org/flashrom/trunk@1739 2b7e53f0-3cfb-0310-b3e9-8179ed1497e1