ARM: dts: stm32: Add KS8851-16MLL ethernet on FMC2

Add DT entries, Kconfig entries and board-specific entries to configure
FMC2 bus and make KS8851-16MLL on that bus accessible to U-Boot.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Patrick Delaunay <patrick.delaunay@st.com>
Cc: Patrice Chotard <patrice.chotard@st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@st.com>
Reviewed-by: Patrick Delaunay <patrick.delaunay@st.com>
diff --git a/configs/stm32mp15_dhcom_basic_defconfig b/configs/stm32mp15_dhcom_basic_defconfig
index 921dea2..683f15e 100644
--- a/configs/stm32mp15_dhcom_basic_defconfig
+++ b/configs/stm32mp15_dhcom_basic_defconfig
@@ -93,6 +93,7 @@
 CONFIG_SPL_SPI_FLASH_MTD=y
 CONFIG_DM_ETH=y
 CONFIG_DWC_ETH_QOS=y
+CONFIG_KS8851_MLL=y
 CONFIG_PHY=y
 CONFIG_PHY_STM32_USBPHYC=y
 CONFIG_PINCONF=y