The CAT875 sub board from Silicon Linux uses a Realtek PHY.

The phy driver commit bbc4d71d ("net: phy: realtek: fix
rtl8211e rx/tx delay config") introduced NFS mount failures.  Now it
needs both rx/tx delays for the NFS mount to work.

This patch fixes the NFS mount failure issue by adding "rgmii-id" mode
to the avb device node.
Signed-off-by: Biju Das <biju.das.jz@bp.renesas.com>
Fixes: bbc4d71d ("net: phy: realtek: fix rtl8211e rx/tx delay config")
Link: https://lore.kernel.org/r/20211115142830.12651-1-biju.das.jz@bp.renesas.comSigned-off-by: Geert Uytterhoeven <geert+renesas@glider.be>