gru: add debug option for cache flushing

Add a debug option to the SGI GRU driver for flushing GRU cache lines from
memory.  In theory this is not needed but it is useful for debugging.
This has no use by end users.

Signed-off-by: Jack Steiner <steiner@sgi.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>