[POWERPC] Split out the logic that allocates struct iommus

Split out the logic that allocates a struct iommu into a separate
function.  This can fail however the calling code has never cared - so
just return if we can't allocate an iommu.

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