igb_uio: add write combining option

igb_uio: add write combining option

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

To get internal_addr memory need to be mapped. But as memory could not be
mapped twice: with and without WC it should be skipped for WC. [1]

To do not spoil other drivers that potentially could use internal_addr,
parameter wc_activate adds possibility to skip it for those PMDs,
that do not use it.

[1] https://www.kernel.org/doc/ols/2008/ols2008v2-pages-135-144.pdf
	section 5.3 and 5.4

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