eal/linux: enable uio_pci_generic support

eal/linux: enable uio_pci_generic support

Change the EAL PCI code so that it can work with both the
uio_pci_generic in-tree driver, as well as the igb_uio
DPDK-specific driver.

This involves changes to
1) Modify method of retrieving BAR resource mapping information
2) Mapping using resource files in /sys rather than /dev/uio*
2) Setup bus master bit in NIC's PCIe configuration space for
uio_pci_generic.

Signed-off-by: Danny Zhou <danny.zhou@intel.com>
Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
Acked-by: Declan Doherty <declan.doherty@intel.com>