MIPS: u-boot.lds: introduce symbol __image_copy_end

This symbol is used in later patches as end address
for relocation of the U-Boot image into RAM.

Signed-off-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>

arch/mips/cpu/u-boot.lds

@@ -70,6 +70,8 @@ SECTIONS
 	uboot_end_data = .;
 
 	. = ALIGN(4);
+	__image_copy_end = .;
+
 	.bss : {
 		__bss_start = .;
 		*(.sbss.*)