powerpc/pci: Add PCI resource alignment documentation

In order to enable SRIOV on PowerNV platform, the PF's IOV BAR needs to be
adjusted:

    1. size expanded
    2. aligned to M64BT size

This patch documents this change on the reason and how.

[bhelgaas: reformat, clarify, expand]
Signed-off-by: Wei Yang <weiyang@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>