net: thunderx: Add RGMII interface type support

This patch adds RGX/RGMII interface type support to BGX
driver. This type of interface is supported by 81xx SOC.

CN81XX VNIC has 8 VFs and max possible LMAC interfaces are 9,
hence RGMII interface will not work if all DLMs are in BGX mode
and all 8 LMACs are enabled

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