nios2: define _end in link script

Since commit 44c6e659
"rename _end to __bss_end__" , the _end was removed.
But we need it now for separated device tree control,
ie, CONFIG_OF_SEPARATE .

The _end is used by fdtdec_setup() to find the blob.
Signed-off-by: NThomas Chou <thomas@wytron.com.tw>
Acked-by: NMarek Vasut <marex@denx.de>
Reviewed-by: NSimon Glass <sjg@chromium.org>

arch/nios2/cpu/u-boot.lds

@@ -67,6 +67,12 @@ SECTIONS
 	_edata = .;
 	PROVIDE (edata = .);
 
+	/*
+	 * _end - This is end of u-boot.bin image.
+	 * dtb will be appended here to make u-boot-dtb.bin
+	 */
+	_end = .;
+
 	/* UNINIT DATA - Small uninitialized data is first so it's
 	 * adjacent to sdata and can be referenced via gp. The normal
 	 * bss follows. We keep it adjacent to simplify init code.