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: NRick Chen <rick@andestech.com>
Signed-off-by: NRick Chen <rickchen36@gmail.com>

common/image.c

@@ -86,6 +86,7 @@ static const table_entry_t uimage_arch[] = {
 	{	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,			"",		"",		},
 };