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>

arch/riscv/cpu/fu540/Makefile

@@ -3,5 +3,9 @@
 # Copyright (C) 2020 SiFive, Inc
 # Pragnesh Patel <pragnesh.patel@sifive.com>
 
+ifeq ($(CONFIG_SPL_BUILD),y)
+obj-y += spl.o
+else
 obj-y += dram.o
 obj-y += cpu.o
+endif