riscv: sifive: fu540: Enable GEMGXL MGMT driver

Enable the new GEMGXL MGMT driver so that GEM 10/100 Mbps works now.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Lukas Auer <lukas.auer@aisec.fraunhofer.de>
Tested-by: Lukas Auer <lukas.auer@aisec.fraunhofer.de>
diff --git a/board/sifive/fu540/Kconfig b/board/sifive/fu540/Kconfig
index f464379..8eb5e30 100644
--- a/board/sifive/fu540/Kconfig
+++ b/board/sifive/fu540/Kconfig
@@ -28,6 +28,7 @@
 	imply CMD_PING
 	imply CLK_SIFIVE
 	imply CLK_SIFIVE_FU540_PRCI
+	imply CLK_SIFIVE_GEMGXL_MGMT
 	imply DOS_PARTITION
 	imply EFI_PARTITION
 	imply IP_DYN