docs: move sparc documentation under Documentation/arch/

Architecture-specific documentation is being moved into Documentation/arch/
as a way of cleaning up the top-level documentation directory and making
the docs hierarchy more closely match the source hierarchy.  Move
Documentation/sparc into arch/ and fix all in-tree references.

Cc: "David S. Miller" <davem@davemloft.net>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>