ARM: dts: imx7d-zii-rmu2: fix rgmii phy-mode for ksz9031 phy

Since commit bcf3440c ("net: phy: micrel: add phy-mode support for the
KSZ9031 PHY") the networking is broken on the imx7d-zii-rmu2 board.

The end result is that network receive behaviour is marginal with lots of
RX CRC errors experienced and NFS frequently failing.

Quoting the explanation from Andrew Lunn in commit 0672d22a
("ARM: dts: imx: Fix the AR803X phy-mode"):

"The problem here is, all the DTs were broken since day 0. However,
because the PHY driver was also broken, nobody noticed and it
worked. Now that the PHY driver has been fixed, all the bugs in the
DTs now become an issue"

Fix it by switching to phy-mode = "rgmii-id".

Fixes: bcf3440c ("net: phy: micrel: add phy-mode support for the KSZ9031 PHY")
Signed-off-by: Chris Healy <cphealy@gmail.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>