fbdev: Teaches offb about palette on radeon r5xx/r6xx

The offb driver already has a collection of hacks to be able to set
the palette on various chips. This adds support for r5xx/r6xx radeons.

This is needed as offb is the only console solution on these currently
and the firmware in some cases sets a really bad color palette. This
fixes using some Radeon X16xx on the Powerstation for example.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>