devlink: Support querying hardware address of port function
Add support to query the hardware address of function represented
by devlink port function.
Example of a PCI VF port which supports a port function:
$ devlink port show pci/0000:06:00.0/2
pci/0000:06:00.0/2: type eth netdev enp6s0pf0vf1 flavour pcivf pfnum 0 vfnum 1
function:
hw_addr 00:11:22:33:44:66