arm64: dts: imx8mm-kontron: Fix connection type for VSC8531 RGMII PHY

Previously we falsely relied on the PHY driver to unconditionally
enable the internal RX delay. Since the following fix for the PHY
driver this is not the case anymore:

commit 7b005a17 ("net: phy: mscc: configure both RX and TX internal
delays for RGMII")

In order to enable the delay we need to set the connection type to
"rgmii-rxid". Without the RX delay the ethernet is not functional at
all.

Fixes: 8668d8b2

 ("arm64: dts: Add the Kontron i.MX8M Mini SoMs and baseboards")
Cc: stable@vger.kernel.org
Signed-off-by: Frieder Schrempf <frieder.schrempf@kontron.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>