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/cpu/generic/Kconfig b/arch/riscv/cpu/generic/Kconfig
index b7552f5..b2cb155 100644
--- a/arch/riscv/cpu/generic/Kconfig
+++ b/arch/riscv/cpu/generic/Kconfig
@@ -10,3 +10,6 @@
 	imply RISCV_TIMER
 	imply SIFIVE_CLINT if (RISCV_MMODE || SPL_RISCV_MMODE)
 	imply CMD_CPU
+	imply SPL_CPU_SUPPORT
+	imply SPL_OPENSBI
+	imply SPL_LOAD_FIT