net: cadence: macb: add EEE LPI statistics counters

Expose the GEM MAC's EEE Low Power Idle hardware counters through
ethtool -S:

- rx_lpi_transitions: number of RX LPI entry events
- rx_lpi_time: cumulative time spent in RX LPI
- tx_lpi_transitions: number of TX LPI entry events (TXLPIEN 0->1)
- tx_lpi_time: cumulative time in TX LPI

These are clear-on-read hardware registers at offsets 0x270-0x27c,
automatically collected by the existing gem_statistics read loop.

Signed-off-by: Nicolai Buchwitz <nb@tipi-net.de>

Author: nbuchwitz

drivers/net/ethernet/cadence/macb.h

@@ -1047,6 +1047,10 @@ struct gem_stats {
 	u32	rx_ip_header_checksum_errors;
 	u32	rx_tcp_checksum_errors;
 	u32	rx_udp_checksum_errors;
+	u32	rx_lpi_transitions;
+	u32	rx_lpi_time;
+	u32	tx_lpi_transitions;
+	u32	tx_lpi_time;
 };
 
 /* Describes the name and offset of an individual statistic register, as
@@ -1146,6 +1150,10 @@ static const struct gem_statistic gem_statistics[] = {
 			    GEM_BIT(NDS_RXERR)),
 	GEM_STAT_TITLE_BITS(RXUDPCCNT, "rx_udp_checksum_errors",
 			    GEM_BIT(NDS_RXERR)),
+	GEM_STAT_TITLE(RXLPI, "rx_lpi_transitions"),
+	GEM_STAT_TITLE(RXLPITIME, "rx_lpi_time"),
+	GEM_STAT_TITLE(TXLPI, "tx_lpi_transitions"),
+	GEM_STAT_TITLE(TXLPITIME, "tx_lpi_time"),
 };
 
 #define GEM_STATS_LEN ARRAY_SIZE(gem_statistics)