Mostly IO modifications, to increase compatibility with FreeBSD.

Mostly IO modifications, to increase compatibility with FreeBSD.

dev/pcidev.cc:
    Allow writes to some PCI read-only registers.
    Fix problem when writing to a zero offset IO location.
dev/tsunami_io.cc:
    Fix calculation of IO addresses.
    Add registers for keyboard and PCI DMA.
dev/tsunamireg.h:
    Add registers for keyboard and PCI DMA.
python/m5/objects/System.py:
    Allow generic System to be instantiated.

--HG--
extra : convert_revision : 1b985ffa2b8e15aa55246f1d14da615c32ecd3f9