ARM: dts: at91: sama5d3_xplained: change phy-mode

Since commit bcf3440c ("net: phy: micrel: add phy-mode support for the
KSZ9031 PHY"), networking is broken on sama5d3 xplained.

The device tree has phy-mode = "rgmii" and this worked before, because
KSZ9031 PHY started with default RGMII internal delays configuration (TX
off, RX on 1.2 ns) and MAC provided TX delay. After above commit, the
KSZ9031 PHY starts handling phy mode properly and disables RX delay, as
result networking is become broken.

Fix it by switching to phy-mode = "rgmii-rxid" to reflect previous
behavior.

Fixes: bcf3440c ("net: phy: micrel: add phy-mode support for the KSZ9031 PHY")
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20200717233644.841080-1-alexandre.belloni@bootlin.com