arm64: tegra: Fix ethernet phy-mode for Jetson Xavier

The 'phy-mode' property is currently defined as 'rgmii' for Jetson
Xavier. This indicates that the RGMII RX and TX delays are set by the
MAC and the internal delays set by the PHY are not used.

If the Marvell PHY driver is enabled, such that it is used and not the
generic PHY, ethernet failures are seen (DHCP is failing to obtain an
IP address) and this is caused because the Marvell PHY driver is
disabling the internal RX and TX delays. For Jetson Xavier the internal
PHY RX and TX delay should be used and so fix this by setting the
'phy-mode' to 'rgmii-id' and not 'rgmii'.

Fixes: f89b58ce ("arm64: tegra: Add ethernet controller on Tegra194")
Signed-off-by: Jon Hunter <jonathanh@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>