docs: arm64: convert perf.txt to ReST format

This file is almost in ReST. All it needs is a rename and
adding a :field: for the two fields at the beginning
(author and date).

While here, add a proper SPDX header, and use the standard
markup for document titles, just for consistency.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/c99bebf166559e9098a9eb78fb2eab2847fffb05.1592203650.git.mchehab+huawei@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/arm64/index.rst

@@ -14,6 +14,7 @@ ARM64 Architecture
     hugetlbpage
     legacy_instructions
     memory
+    perf
     pointer-authentication
     silicon-errata
     sve

Documentation/arm64/perf.rst

@@ -1,8 +1,11 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+=====================
 Perf Event Attributes
 =====================
 
-Author: Andrew Murray <andrew.murray@arm.com>
-Date: 2019-03-06
+:Author: Andrew Murray <andrew.murray@arm.com>
+:Date: 2019-03-06
 
 exclude_user
 ------------