Allow chipset drivers to specify the direct-mapped DMA window's mask in

Allow chipset drivers to specify the direct-mapped DMA window's mask in
preparation for tsunami support.  Previous chipsets' direct-mapped DMA
mask was always 1024*1024*1024.  The Tsunami chipset needs it to be
2*1024*1024*1024

These changes should not affect the i386 port

Reviewed by:	Doug Rabson <dfr@nlsystems.com>