documentation: fix almost duplicate filenames (IO/io-mapping.txt)

Having both IO-mapping.txt and io-mapping.txt in Documentation/
was confusing and/or bothersome to some people, so rename
IO-mapping.txt to bus-virt-phys-mapping.txt.  Also update
Documentation/00-INDEX for both of these files.

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Cc: Kees Bakker <kees.bakker@xs4all.nl>
Cc: Keith Packard <keithp@keithp.com>