viafb: documentation update

We now support the VX855, and the VX800 is no longer unaccellerated.
viafb_video_dev was removed as it was useless.

Signed-off-by: Harald Welte <HaraldWelte@viatech.com>
Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>
Cc: Joseph Chan <JosephChan@via.com.tw>
Cc: Scott Fang <ScottFang@viatech.com.cn>
Cc: Krzysztof Helt <krzysztof.h1@poczta.fm>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>