add d/copyright file

packaging is ours and open source, but firmware file copyright /
license relations are complex, so just refer to the upstream license
fails for each vendor/firmware.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>

debian/copyright

@@ -0,0 +1,35 @@
+Format: https://www.debian.org/doc/packaging-manuals/copyright-format/1.0/
+Source: https://git.proxmox.com/?p=pve-firmware.git;a=summary
+
+Files:
+ *
+Copyright: 2008 - 2023, Proxmox Server Solutions GmbH <support@proxmox.com>
+License: GPL-2+
+ This program is free software; you can redistribute it and/or modify
+ it under the terms of the GNU General Public License as published by
+ the Free Software Foundation; either version 2 of the License, or
+ (at your option) any later version.
+ .
+ This program is distributed in the hope that it will be useful,
+ but WITHOUT ANY WARRANTY; without even the implied warranty of
+ MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ GNU General Public License for more details.
+ .
+ You should have received a copy of the GNU General Public License
+ along with this package; if not, see &lt;https://www.gnu.org/licenses/&gt;.
+Comment:
+ On Debian systems, the full text of the GNU General Public License
+ version 2 can be found in the file '/usr/share/common-licenses/GPL-2'.
+
+Files:
+ lib/firmware/*
+Copyright: 1994 - 2023, the Linux Firmware Contributors
+License: Mixed
+ As per the linux-firmware README: The lincense files shipped by this package
+ are comming from many different vendors and are licensed in as many different
+ ways, some under free licenses and some under non-free, but with grants for
+ being distributable and including an implicit or explicit patent grant to end
+ users to ensure full functionality of device operation with the firmware.
+Comment:
+ For the various licensesm, see /usr/share/doc/pve-firmware/README.gz and the
+ '/usr/share/doc/pve-firmware/licenses/' directory.