MIPS: define __init_end in u-boot.lds

The generic board code uses the __init_end symbol to calculate
monitor_flash_len. Define said symbol for MIPS, equivalent to
__image_copy_end which is used for the same purpose in
arch/mips/lib/board.c.
Signed-off-by: NPaul Burton <paul.burton@imgtec.com>

arch/mips/cpu/u-boot.lds

@@ -53,6 +53,7 @@ SECTIONS
 
 	. = ALIGN(4);
 	__image_copy_end = .;
+	__init_end = .;
 
 	.rel.dyn : {
 		__rel_dyn_start = .;