fbdev: Add GXT4000P and GXT6500P support to the gxt4500 driver

I'm reviving an old patch from 2009 that adds support for GXT4000P and GXT6500P
adapter to the gxt4500 driver.

See threads at http://marc.info/?l=linux-fbdev-devel&m=124345080216952&w=2
and https://lists.ozlabs.org/pipermail/linuxppc-dev/2009-June/072672.html
for more details.

This patch adds support for GXT4000P and GXT6500P cards found on some
IBM pSeries machines.
GXT4000P/6000P and GXT4500P/6500P  couples are  identical from
software's point of view and are based on the same  Raster Engine
(RC1000), except for a different reference clock for the PLL.
GXT6x00P models are equipped with an additional Geometry Engine
(GT1000) but this driver doesn't use it.

Signed-off-by: Nico Macrionitis <acrux@cruxppc.org>
Signed-off-by: Giuseppe Coviello <cjg@cruxppc.org>
Tested-by: Dan Horák <dan@danny.cz>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>