ARM: 8996/1: Documentation/Clean up the description of mach-<class>

Polish the description of machine classes a little bit,
remove the duplicate directory name, so the reader does
not feel compelled to dig through the output of "git blame".

Signed-off-by: Pete Zaitcev <zaitcev@yahoo.com>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>