riscv: Add _image_binary_end for SPL

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>