Add support for userland to access I/O port space. This is primarily
added for XFree86. There are 2 reasons for doing this with sysarch():

1. The memory mapped I/O space is not at a fixed physical address. An application has to use some interface to get the base address. It gets worse if the machine has multiple memory mapped I/O spaces.
2. Access to the memory mapped I/O space needs to happen through a translation that is flagged as uncachable. There's no interface that allows a process to do uncached memory I/O, other than though /dev/mem (possibly).

So, until we either disallow direct access to I/O or bus space from
userland or have a better way of doing this, sysarch() has the least
negative impact on existing interfaces.