powerpc/pseries/ddw: Extend upper limit for huge DMA window for persistent memory

powerpc/pseries/ddw: Extend upper limit for huge DMA window for persistent memory

Unlike normal memory ("memory" compatible type in the FDT), the
persistent memory ("ibm,pmemory" in the FDT) can be mapped anywhere in
the guest physical space and it can be used for DMA.

In order to maintain 1:1 mapping via the huge DMA window, we need to
know the maximum physical address at the time of the window setup. So
far we've been looking at "memory" nodes but "ibm,pmemory" does not
have fixed addresses and the persistent memory may be mapped
afterwards.

Since the persistent memory is still backed with page structs, use
MAX_PHYSMEM_BITS as the upper limit.

This effectively disables huge DMA window in LPAR under pHyp if
persistent memory is present but this is the best we can do for the
moment.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Tested-by: Wen Xiong<wenxiong@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200331012338.23773-1-aik@ozlabs.ru