powerpc/powernv: Override pcibios_default_alignment() to force PCI devices to be page aligned

Override pcibios_default_alignment() to set default alignment to PAGE_SIZE
for all PCI devices on PowerNV platform.  Thus sub-page BARs would not
share a page and could be mapped into guest when VFIO passthrough them.

Signed-off-by: Yongji Xie <elohimes@gmail.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>