cirrusfb: add accelerator constant

Add an accelerator constant so almost all Cirrus are recognized as
accelerators by the fbset command.

Signed-off-by: Krzysztof Helt <krzysztof.h1@wp.pl>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>