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>

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

@@ -16,6 +16,7 @@ ENTRY(_start)
 SECTIONS
 {
 	. = ALIGN(4);
+	__image_copy_start = ADDR(.text);
 	.text : {
 		arch/riscv/cpu/start.o	(.text)
 		*(.text*)
@@ -46,6 +47,7 @@ SECTIONS
 
 	_end = .;
 	_image_binary_end = .;
+	__image_copy_end = .;
 
 	.bss : {
 		__bss_start = .;

arch/riscv/cpu/u-boot.lds

@@ -10,6 +10,7 @@ ENTRY(_start)
 SECTIONS
 {
 	. = ALIGN(4);
+	__image_copy_start = ADDR(.text);
 	.text : {
 		arch/riscv/cpu/start.o	(.text)
 	}
@@ -57,6 +58,8 @@ SECTIONS
 		__efi_runtime_rel_stop = .;
 	}
 
+	__image_copy_end = .;
+
 	/DISCARD/ : { *(.rela.plt*) }
 	.rela.dyn : {
 		__rel_dyn_start = .;