powerpc/powernv: Reserve additional space for IOV BAR, with m64_per_iov supported

M64 aperture size is limited on PHB3.  When the IOV BAR is too big, this
will exceed the limitation and failed to be assigned.

Introduce a different mechanism based on the IOV BAR size:

  - if IOV BAR size is smaller than 64MB, expand to total_pe
  - if IOV BAR size is bigger than 64MB, roundup power2

[bhelgaas: make dev_printk() output more consistent, use PCI_SRIOV_NUM_BARS]
Signed-off-by: Wei Yang <weiyang@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>