imx8mm-cl-iot-gate: Add support for the Realtek RTL8211E PHY

Newer imx8mm-cl-iot-gate versions are populated with a Realtek RTL8211E
PHY instead of the Atheros AR8033.

Adapted Compulab's patch from:
https://github.com/compulab-yokneam/meta-bsp-imx8mm/blob/iot-gate-imx8_5.10.72/recipes-bsp/u-boot/compulab/imx8mm/0125-imx8mm-net-enable-phy-Realtek-RTL8211E.patch

to support both PHYs in U-Boot.

Signed-off-by: Fabio Estevam <festevam@denx.de>
diff --git a/include/configs/imx8mm-cl-iot-gate.h b/include/configs/imx8mm-cl-iot-gate.h
index 09d87cf..0c54702 100644
--- a/include/configs/imx8mm-cl-iot-gate.h
+++ b/include/configs/imx8mm-cl-iot-gate.h
@@ -136,7 +136,7 @@
 #define CFG_SYS_FSL_USDHC_NUM	2
 #define CFG_SYS_FSL_ESDHC_ADDR	0
 
-#define CFG_FEC_MXC_PHYADDR		0
+#define CFG_FEC_MXC_PHYADDR		-1 /* Auto search of PHY on MII */
 
 /* USB Configs */
 #define CFG_MXC_USB_PORTSC	(PORT_PTS_UTMI | PORT_PTS_PTW)