powerpc/fsl_msi: add support for "msi-address-64" property

Add support for the msi-address-64 property of a PCI node.  This property
specifies the PCI address of MSIIR (message signaled interrupt index
register).

In commit 3da34aae ("powerpc/fsl: Support unique MSI addresses per PCIe Root
Complex"), the msi_addr_hi/msi_addr_lo fields of struct fsl_msi were redefined
from an actual address to just an offset, but the fields were not renamed
accordingly.  These fields are replace with a single field, msiir_offset,
to reflect the new meaning.

Signed-off-by: Timur Tabi <timur@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>