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: Thomas Chou <thomas@wytron.com.tw>
Acked-by: Marek Vasut <marex@denx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>