ArmVirtPkg: add "Contributions.txt" and "License.txt"

Both files originate from ArmPlatformPkg.

The copyright notices at the top of "License.txt" have been refreshed as
follows:
- I grepped ArmVirtPkg for "copyright", case-insensitively
- for each copyright holder company, I unified the intervals to the
  strictest superset (no absent year was introduced in this process)
- I listed the copyright notices in decreasing order of last contribution
  year.
- When a "last contribution year" was shared by several notices, I sorted
  those between each other in descending "first contribution year" order.

Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Olivier Martin <olivier.martin@arm.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Andrew Fish <afish@apple.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>

git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@17582 6f19259b-4bc3-4df7-8a09-765794883524