net/nfp: support vNIC Rx/Tx BAR mappings

net/nfp: support vNIC Rx/Tx BAR mappings

NFP vNICs use a subset of PCI device BARs. vNIC rx/tx bars point to
NFP hardware queues unit. Unlike vNIC config bar, the NFP address is
always the same so the NFP expansion bar configuration always uses
the same hardcoded physical address.

This patch adds a NSPU API function for getting vNIC rx/tx bars
mapped through a expansion bar using that specific physical address.

The PMD will use the PCI bar offset returned for mapping the vNIC
rx/tx bars.

Signed-off-by: Alejandro Lucero <alejandro.lucero@netronome.com>