net: stmmac: mmc_core: Add GMAC LPI statistics

XGMAC MMC has already added LPI statistics. GMAC MMC lack of these
statistics. Add register definition and reading the LPI statistics
from registers.

Signed-off-by: Minda Chen <minda.chen@starfivetech.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>