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>