riscv: add SPL support

U-Boot SPL on the generic RISC-V CPU supports two boot flows, directly
jumping to the image and via OpenSBI firmware. In the first case, both
U-Boot SPL and proper must be compiled to run in the same privilege
mode. Using OpenSBI firmware, U-Boot SPL must be compiled for machine
mode and U-Boot proper for supervisor mode.

To be able to use SPL, boards have to provide a supported SPL boot
device.

Signed-off-by: Lukas Auer <lukas.auer@aisec.fraunhofer.de>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Anup Patel <anup.patel@wdc.com>
diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
index b8d01ba..01975d7 100644
--- a/arch/riscv/Kconfig
+++ b/arch/riscv/Kconfig
@@ -226,4 +226,7 @@
 	int
 	default 13
 
+config SPL_LDSCRIPT
+	default "arch/riscv/cpu/u-boot-spl.lds"
+
 endmenu