riscv: Add _image_binary_end for SPL

For SPL_SEPARATE_BSS, Device tree will be put at _image_binary_end

Signed-off-by: Pragnesh Patel <pragnesh.patel@sifive.com>
Reviewed-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Jagan Teki <jagan@amarulasolutions.com>

arch/riscv/cpu/u-boot-spl.lds

@@ -72,6 +72,7 @@ SECTIONS
 	. = ALIGN(4);
 
 	_end = .;
+	_image_binary_end = .;
 
 	.bss : {
 		__bss_start = .;