bus/pci: enable write combining during mapping

bus/pci: enable write combining during mapping

Write combining (WC) increases NIC performance by making better
utilization of PCI bus, but cannot be used by all PMDs.

It will be enabled only if RTE_PCI_DRV_WC_ACTIVATE will be set in
drivers flags. For proper work also igb_uio driver must be loaded with
wc_activate set to 1.

When mapping PCI resources, firstly check if it support WC
and then try to use it.
In case of failure, it will fallback to normal mode.

Signed-off-by: Rafal Kozik <rk@semihalf.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>