ravb: Initialize GbEthernet E-MAC

Initialize GbEthernet E-MAC found on RZ/G2L SoC.
This patch also renames ravb_set_rate to ravb_set_rate_rcar and
ravb_rcar_emac_init to ravb_emac_init_rcar to be consistent with
the naming convention used in sh_eth driver.

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