net: thunderx: Support for 83xx mixed QLM/DLM config

83xx has 4 BGX blocks and are enabled mixed QLM/DLM
configs. BGX0/BGX1 are from QLM2/QLM3, BGX3 is DLM4
and BGX2 is split across DLM5 & DLM6.

This patch adds support for BGX2's split config and also
enables all 4 BGXs to be used.

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