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 <[e-mail]>
Cc: Kees Bakker <[e-mail]>
Cc: Keith Packard <[e-mail]>

Documentation/00-INDEX

···
 	- directory with DocBook templates etc. for kernel documentation.
 HOWTO
 	- the process and procedures of how to do Linux kernel development.
-IO-mapping.txt
-	- how to access I/O mapped memory from within device drivers.
 IPMI.txt
 	- info on Linux Intelligent Platform Management Interface (IPMI) Driver.
 IRQ-affinity.txt
···
 	- info on block devices & drivers
 btmrvl.txt
 	- info on Marvell Bluetooth driver usage.
+bus-virt-phys-mapping.txt
+	- how to access I/O mapped memory from within device drivers.
 cachetlb.txt
 	- describes the cache/TLB flushing interfaces Linux uses.
 cdrom/
···
 	- how to use the RAM disk as an initial/temporary root filesystem.
 input/
 	- info on Linux input device support.
+io-mapping.txt
+	- description of io_mapping functions in linux/io-mapping.h
 io_ordering.txt
 	- info on ordering I/O writes to memory-mapped addresses.
 ioctl/