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/Kconfig b/arch/Kconfig
index e510e97..f4ada57 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -76,6 +76,12 @@
 	imply MTD
 	imply TIMER
 	imply CMD_DM
+	imply SPL_DM
+	imply SPL_OF_CONTROL
+	imply SPL_LIBCOMMON_SUPPORT
+	imply SPL_LIBGENERIC_SUPPORT
+	imply SPL_SERIAL_SUPPORT
+	imply SPL_TIMER
 
 config SANDBOX
 	bool "Sandbox"