OvmfPkg/License.txt: refresh the MIT license text and include the SPDX ID

OvmfPkg/License.txt: refresh the MIT license text and include the SPDX ID

Refresh the MIT license text from <https://spdx.org/licenses/MIT.html>,
and add "SPDX-License-Identifier: MIT" for easier parsing by machines.

This follows the tree-wide adoption of "SPDX-License-Identifier"s, made
for <https://bugzilla.tianocore.org/show_bug.cgi?id=1373>; in particular:

- commit 304bff7223a8 ("edk2: Change License.txt from 2-Clause BSD to
  BSD+Patent", 2019-04-09),

- commit 75fae505b735 ("OvmfPkg: Change License.txt from 2-Clause BSD to
  BSD+Patent", 2019-04-09).

Cc: Anthony Perard <anthony.perard@citrix.com>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Julien Grall <julien.grall@arm.com>
Cc: Lars Kurth <lars.kurth@citrix.com>
Cc: xen-devel@lists.xenproject.org
Bugzilla: https://bugzilla.tianocore.org/show_bug.cgi?id=1654
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Philippe Mathieu-Daude <philmd@redhat.com>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Anthony PERARD <anthony.perard@citrix.com>