net: thunderx: Support for internal loopback mode

Support for setting VF's corresponding BGX LMAC in internal
loopback mode. This mode can be used for verifying basic HW
functionality such as packet I/O, RX checksum validation,
CQ/RBDR interrupts, stats e.t.c. Useful when DUT has no external
network connectivity.

'loopback' mode can be enabled or disabled via ethtool.

Note: This feature is not supported when no of VFs enabled are
morethan no of physical interfaces i.e active BGX LMACs

Signed-off-by: Sunil Goutham <sgoutham@cavium.com>
Signed-off-by: Aleksey Makarov <aleksey.makarov@caviumnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>