net: stmmac: mmc: Add Frame Preemption counters on GMAC5+ cores

This can be useful for debug. Add these counters on GMAC5+ cores just
like we did for XGMAC.

Signed-off-by: Jose Abreu <Jose.Abreu@synopsys.com>
Signed-off-by: David S. Miller <davem@davemloft.net>