docs: filesystems: convert efivarfs.txt to ReST

Trivial changes:

- Add a SPDX header;
- Adjust document title;
- Mark a literal block as such;
- Add it to filesystems/index.rst.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/215691d747055c4ccb038ec7d78d8d1fe87fe2c0.1581955849.git.mchehab+huawei@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
diff --git a/Documentation/filesystems/index.rst b/Documentation/filesystems/index.rst
index d6d69f1..4230f49 100644
--- a/Documentation/filesystems/index.rst
+++ b/Documentation/filesystems/index.rst
@@ -60,6 +60,7 @@
    debugfs
    dlmfs
    ecryptfs
+   efivarfs
    fuse
    overlayfs
    virtiofs