Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Merge from projects/mips to head by hand:

Merge support for very early alchemy port. I wouldn't merge this
except I don't want it to get lost when we retire projects/mips.
Should be consiered pre-alpha at this stage. Also, alchemy is now
owned by rmi, but started out life as a separate processor line, so
I'm leaving it in its own directory rather than try to shoe-horn it
into the unrelated rmi directory. Its future location is an open
question.

- Make mips_bus_space_generic be of type bus_space_tag_t instead of
struct bus_space and update all relevant places.

Use FreeBSD/arm approach for handling bus space access: space tag is a pointer
to bus_space structure that defines access methods and hence every bus can
define own accessors. Default space is mips_bus_space_generic. It's a simple
interface to physical memory, values are read with regard to host system
byte order.

- Revert changes accidentally killed by merge operation

First milestone on Alchemy port: Simplest kernel now compiles.