[POWERPC] Allocate the hash table under 1G on cell

In order to support the fixed IOMMU mapping (in a subsequent patch),
we need the hash table to be inside the IOMMUs DMA window.  This is
usually 2G, but let's make sure the hash table is under 1G as that
will satisfy the IOMMU requirements and also means the hash table will
be on node 0.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Paul Mackerras <paulus@samba.org>