arch/tile: catch up on various minor cleanups.

None of these changes fix any actual bugs, but are just various cleanups
that fell out along the way.  In particular, some unused #defines and
includes are removed, PREFETCH_STRIDE is added (the default is right for
our shipping chips, but wrong for our next generation), our tile-specific
prefetching code is removed so the (identical) generic prefetching code
can be used instead, a comment is fixed to be proper GPL and not just a
"paste GPL here" token, a "//" comment is converted to "/* */", etc.

Signed-off-by: Chris Metcalf <cmetcalf@tilera.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>