arm64: dts: imx8mm-venice-gw700x: add support for PHY LED's

The GW700x SoM has an onboard DP83867 RGMII GbE PHY that drives two
LED's (LED1 and LED2, skipping LED0). Add the appropriate dt bindings to
allow these PHY LED's to be controlled via a netdev trigger.

Signed-off-by: Tim Harvey <tharvey@gateworks.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>