tools: mkimage: Support RISC-V arch

Add riscv uimage arch to support riscv-linux booting.
It can Convert riscv-linux to image which can be
booted by bootm command.

Signed-off-by: Rick Chen <rick@andestech.com>
Signed-off-by: Rick Chen <rickchen36@gmail.com>
diff --git a/common/image.c b/common/image.c
index f5278a0..e1c50eb 100644
--- a/common/image.c
+++ b/common/image.c
@@ -86,6 +86,7 @@
 	{	IH_ARCH_ARC,		"arc",		"ARC",		},
 	{	IH_ARCH_X86_64,		"x86_64",	"AMD x86_64",	},
 	{	IH_ARCH_XTENSA,		"xtensa",	"Xtensa",	},
+	{	IH_ARCH_RISCV,		"riscv",	"RISC-V",	},
 	{	-1,			"",		"",		},
 };