viafb: add support for the VX855 chipset

Add support for a new VIA integrated graphics chipset, the VX855.

Signed-off-by: HaraldWelte <HaraldWelte@viatech.com>
Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>
Cc: Scott Fang <ScottFang@viatech.com.cn>
Cc: Joseph Chan <JosephChan@via.com.tw>
Cc: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>