radeonfb: Add support for Radeon xpress 200m

Added support for radeon xpress 200m(rs480).  Note that the card doesn't
like dynclk turned on.

Signed-off-by: Johan Henriksson <jhn98032@gmail.com>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: "Antonino A. Daplas" <adaplas@pol.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>