nios2: remove gp assignments in link script

Since we don't use gp for small data with option "-G0",
we use gp as global data pointer. The _gp location is
not needed.

The ALIGN(16) was for gp only. It is removed, so that we
can get correct address of dtb in u-boot-dtb.bin image.

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