radeonfb: add chip definition for RV370 5b63

... which I've found on a Sapphire X550 (Silent).

Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Andreas Herrmann <aherrman@arcor.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>