ravb: Add RZ/G2L MII interface support

EMAC IP found on RZ/G2L Gb ethernet supports MII interface.
This patch adds support for selecting MII interface mode.

Signed-off-by: Biju Das <biju.das.jz@bp.renesas.com>
Reviewed-by: Sergey Shtylyov <s.shtylyov@omp.ru>
Link: https://lore.kernel.org/r/20220914192604.265859-1-biju.das.jz@bp.renesas.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>