amd-xgbe: extend 10Mbps support to MAC version 21H

MAC version 21H supports the 10Mbps speed. So, extend support to
platforms that support it.

Acked-by: Shyam Sundar S K <Shyam-sundar.S-k@amd.com>
Reviewed-by: Sridhar Samudrala <sridhar.samudrala@intel.com>
Signed-off-by: Raju Rangoju <Raju.Rangoju@amd.com>
Signed-off-by: David S. Miller <davem@davemloft.net>