riscv: sifive: fu540: add SPL configuration

Add a support for SPL which will boot from L2 LIM (0x0800_0000) and
then SPL will boot U-Boot FIT image (OpenSBI FW_DYNAMIC + u-boot.bin)
from MMC boot devices.

SPL related code is leveraged from FSBL
(https://github.com/sifive/freedom-u540-c000-bootloader.git)

Signed-off-by: Pragnesh Patel <pragnesh.patel@sifive.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>
Tested-by: Jagan Teki <jagan@amarulasolutions.com>
diff --git a/board/sifive/fu540/Makefile b/board/sifive/fu540/Makefile
index 6e1862c..b05e2f5 100644
--- a/board/sifive/fu540/Makefile
+++ b/board/sifive/fu540/Makefile
@@ -3,3 +3,7 @@
 # Copyright (c) 2019 Western Digital Corporation or its affiliates.
 
 obj-y	+= fu540.o
+
+ifdef CONFIG_SPL_BUILD
+obj-y += spl.o
+endif