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>
diff --git a/arch/riscv/cpu/u-boot-spl.lds b/arch/riscv/cpu/u-boot-spl.lds
index 9070946..0717833 100644
--- a/arch/riscv/cpu/u-boot-spl.lds
+++ b/arch/riscv/cpu/u-boot-spl.lds
@@ -16,6 +16,7 @@
 SECTIONS
 {
 	. = ALIGN(4);
+	__image_copy_start = ADDR(.text);
 	.text : {
 		arch/riscv/cpu/start.o	(.text)
 		*(.text*)
@@ -46,6 +47,7 @@
 
 	_end = .;
 	_image_binary_end = .;
+	__image_copy_end = .;
 
 	.bss : {
 		__bss_start = .;