riscv: Provide __image_copy_{start_end} symbols in linkerscript

riscv: Provide __image_copy_{start_end} symbols in linkerscript

Binman looks for __image_copy_start to determine the base address of an
entry if elf-base-sym isn't specified, which is missing in RISC-V port.
This causes binman skips RISC-V SPL entries without filling addresses
into its .binman_sym_table section.

This patch defines __image_copy_start in linkerscript of both SPL and
proper U-Boot to ensure binman_sym functions correctly with the default
binman.dtsi. The paired symbol, __image_copy_end, is introduced as well
for completeness.

Signed-off-by: Yao Zi <ziyao@disroot.org>
Reviewed-by: Simon Glass <sjg@chromium.org>