ravb: Add fixed-link support

This patch adds support of the fixed PHY.
This patch is based on commit 87009814 ("ucc_geth: use the new fixed
PHY helpers").

Signed-off-by: Kazuya Mizuguchi <kazuya.mizuguchi.ks@renesas.com>
Signed-off-by: Yoshihiro Kaneko <ykaneko0929@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>