gru: function to generate chipset IPI values

Create a function to generate the value that is written to the UV hub MMR
to cause an IPI interrupt to be sent.  The function will be used in the
GRU message queue error recovery code that sends IPIs to nodes in remote
partitions.

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>