viafb: MAINTAINERS entry

Add maintainers for VIA UniChrome(Pro)/Chrome9 Framebuffer driver

Signed-off-by: Joseph Chan <josephchan@via.com.tw>
Signed-off-by: Scott Fang <ScottFang@viatech.com.cn>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>