ravb: Add half_duplex to struct ravb_hw_info

RZ/G2L supports half duplex mode.
Add a half_duplex hw feature bit to struct ravb_hw_info for
supporting half duplex mode for RZ/G2L.

Signed-off-by: Biju Das <biju.das.jz@bp.renesas.com>
Reviewed-by: Sergey Shtylyov <s.shtylyov@omp.ru>
Signed-off-by: David S. Miller <davem@davemloft.net>