ravb: Add tx_counters to struct ravb_hw_info

The register for retrieving TX counters is present only on R-Car Gen3
and RZ/G2L; it is not present on R-Car Gen2.

Add the tx_counters hw feature bit to struct ravb_hw_info, to enable this
feature specifically for R-Car Gen3 now and later extend it to RZ/G2L.

Signed-off-by: Biju Das <biju.das.jz@bp.renesas.com>
Reviewed-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>