PCI: support sizing P2P bridge I/O windows with 1K granularity

Some bridges support I/O windows with 1K alignment, not just the 4K
alignment defined by the PCI spec.  For example, see the IOBL_ADR register
and the EN1K bit in the CNF register in the Intel 82870P2 (P64H2).

This patch adds support for sizing the window in 1K increments based
on the requirements of downstream devices.

[bhelgaas: changelog, comment]
Signed-off-by: Yinghai Lu <yinghai@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>