riscv: image: Add new image type for RV64

Similar to ARM and X86, introduce a new image type which allows u-boot
to distinguish between images built for 32-bit vs 64-bit Risc-V CPUs.

Signed-off-by: Mayuresh Chitale <mchitale@ventanamicro.com>
Reviewed-by: Maxim Moskalets <maximmosk4@gmail.com>

include/image.h

@@ -138,7 +138,8 @@ enum {
 	IH_ARCH_ARC,			/* Synopsys DesignWare ARC */
 	IH_ARCH_X86_64,			/* AMD x86_64, Intel and Via */
 	IH_ARCH_XTENSA,			/* Xtensa	*/
-	IH_ARCH_RISCV,			/* RISC-V */
+	IH_ARCH_RISCV,			/* RISC-V 32 bit*/
+	IH_ARCH_RISCV64,		/* RISC-V 64 bit*/
 
 	IH_ARCH_COUNT,
 };