RMDA/siw: Require a 64 bit arch

The new siw driver fails to build on i386 with

drivers/infiniband/sw/siw/siw_qp.c:1025:3: error: invalid output size for constraint '+q'
                smp_store_mb(*cq->notify, SIW_NOTIFY_NOT);

As it is using 64 bit values with the smp_store_mb.

Since the entire scheme here seems questionable, and we are in the merge
window, fix the compile failures by disabling 32 bit support on this
driver.

A proper fix will be reviewed post merge window.

Fixes: c0cf5bdd

 ("rdma/siw: addition to kernel build environment")
Reported-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>