powerpc/pseries/iommu: Create huge DMA window if no MMIO32 is present

The iommu_init_table() helper takes an address range to reserve in
the IOMMU table being initialized to exclude MMIO addresses, this is
useful if the window stretches far beyond 4GB (although wastes some TCEs).
At the moment the code searches for such MMIO32 range and fails if none
found which is considered a problem while it really is not: it is actually
better as this says there is no MMIO32 to reserve and we can use
usually wasted TCEs. Furthermore PHYP never actually allows creating
windows starting at busaddress=0 so this MMIO32 range is never useful.

This removes error exit and initializes the table with zero range if
no MMIO32 is detected.

Fixes: 381ceda8 ("powerpc/pseries/iommu: Make use of DDW for indirect mapping")
Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20211020132315.2287178-5-aik@ozlabs.ru