allocates kva space from the top down for the device mappings and builds
entries in an internal table which is automatically used later by
arm_devmap_bootstrap(). The platform code just calls the new
arm_devmap_add_entry() function as many times as it needs to (up to 32
entries allowed; most platforms use 2 or 3 at most).
There is also a new arm_devmap_lastaddr() function that returns the lowest
kva address allocated; this can be used to implement initarm_lastaddr()
which is used to initialize vm_max_kernel_address.
The new code is based on a similar concept developed for the imx family
SoCs recently. They will soon be converted to use this new common code.
new devmap.[ch] files. Emphasize the MD nature of these things by using
the prefix arm_devmap_ on the function and type names (already a few of
these things found their way into MI code, hopefully it will be harder to
do by accident in the future).
out common code related to mapping device memory into a new devmap.c file.
Remove the growing duplication of code that used pmap_devmap_find_pa() and
then did some math with the returned results to generate a virtual address,
and likewise in reverse to get a physical address. Now there are a pair
of functions, arm_devmap_vtop() and arm_devmap_ptov(), to do that. The
bus_space_map() implementations are rewritten in terms of these.
