Radeonfb Xpress 200M RC410 support

Make radeonfb work ith the 200m Xpress RC410.  In my tests it was terribly
unstable and would freeze until I set a refresh rate in the kernel argument
to 75.

e.g video=radeonfb:1280x800@75

Now it is rock solid.

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>