riscv: Move do_reset() to a common place

We don't have a reset method on any RISC-V board yet. Instead of
adding the same 'unsupported' message for each CPU variant it might
make more sense to add a generic do_reset function for all CPU
variants to lib/, similar to the one for ARM (arch/arm/lib/reset.c).

Suggested-by: Lukas Auer <lukas.auer@aisec.fraunhofer.de>
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Lukas Auer <lukas.auer@aisec.fraunhofer.de>
diff --git a/arch/riscv/lib/Makefile b/arch/riscv/lib/Makefile
index cc562f9..b58db89 100644
--- a/arch/riscv/lib/Makefile
+++ b/arch/riscv/lib/Makefile
@@ -10,6 +10,7 @@
 obj-$(CONFIG_CMD_GO) += boot.o
 obj-y	+= cache.o
 obj-y	+= interrupts.o
+obj-y	+= reset.o
 obj-y   += setjmp.o
 
 # For building EFI apps