cirrusfb: add mmio registers for Laguna chipsets

The Laguna chipsets use special registers which are available through the
mmio area.  The cirrusfb driver does not use memory mapped registers for
the PCI cards.

Add the memory mapped area for Laguna chipsets and add basic usage of the
special Laguna registers after SVGALIB code.

This gives readable console at 16bpp on the GD-5465 (Laguna AGP).  The
8bpp and 32bpp depths are still broken.

Signed-off-by: Krzysztof Helt <krzysztof.h1@wp.pl>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>