docs: nios2: add missing ReST file

changeset ed13a92d ("docs: archis: add a per-architecture features list")
besides having a typo on its title, it was missing the feature file.

Add it.
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Fixes: ed13a92d ("docs: archis: add a per-architecture features list")
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/e51c4692c4420d28bca35f553a9a3f3d78404d99.1607331056.git.mchehab+huawei@kernel.orgSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/nios2/features.rst

+.. SPDX-License-Identifier: GPL-2.0
+
+.. kernel-feat:: $srctree/Documentation/features nios2