configs: fu540: Add config options for U-Boot SPL

With sifive_fu540_defconfig:

User can use FSBL or u-boot-spl.bin anyone at a time.

For FSBL,
fsbl->fw_payload.bin (opensbi + U-Boot)

For u-boot-spl.bin,
u-boot-spl.bin->FIT image (opensbi + U-Boot proper + dtb)

U-Boot SPL will be loaded by ZSBL from SD card (replace fsbl.bin with
u-boot-spl.bin) and runs in L2 LIM in machine mode and then load FIT
image u-boot.itb from SD card into RAM.

U-Boot SPL expects u-boot.itb FIT image at the starting of SD card sector
number (0x822) of GUID type "2E54B353-1271-4842-806F-E436D6AF6985"

Signed-off-by: Pragnesh Patel <pragnesh.patel@sifive.com>
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>