sifive-fu540: config: Add mmc0 as a boot target device

Add the mmc0 device as a BOOT_TARGET_DEVICES.

Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Lukas Auer <lukas.auer@aisec.fraunhofer.de>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>

include/configs/sifive-fu540.h

@@ -26,6 +26,7 @@
 #define CONFIG_ENV_SIZE			SZ_128K
 
 #define BOOT_TARGET_DEVICES(func) \
+	func(MMC, mmc, 0) \
 	func(DHCP, dhcp, na)
 
 #include <config_distro_bootcmd.h>