[AGPGART] uninorth: Add module param 'aperture' for aperture size

In contrast to most if not all PC BIOSes, OpenFirmware (OF) on PowerMacs with
UniNorth bridges does not allow changing the aperture size. The size set up by
OF is usually 16 MB, which is too low for graphics intensive environments.
Hence, add a module parameter that allows changing the aperture size at driver
initialization time. When the parameter is not specified, the default is 32 MB.

Signed-off-by: Michel Dänzer <michel@tungstengraphics.com>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Dave Jones <davej@redhat.com>