We add CONTRIBUTORS.md file to provide a list of individuals and
organizations actively contributing to the OpenSBI project.

Signed-off-by: Anup Patel <anup.patel@wdc.com>

Also make sure that this file contains only the official BSD 2-clause
license text, nothing else.

Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>

This patch updates copyright header in all files as follows:
1. Makes "SPDX-License-Identifier: BSD-2-Clause" as first line
2. Change copyright year to 2019 for Western Digital

Signed-off-by: Anup Patel <anup.patel@wdc.com>

Signed-off-by: Anup Patel <anup.patel@wdc.com>