sparc32: sun4m interrupt mask cleanup

Here is an updated version of a patch I wrote 6 years ago
http://marc.info/?l=linux-sparc&m=103939103607617&w=2
that simplifies interrupt mask lookup.  It's main purpose
is to add VME bus support but it's really a cleanup of the mask code.

Signed-off-by: Robert Reif <reif@earthlink.net>
Signed-off-by: David S. Miller <davem@davemloft.net>