package/edk2-platforms: refine license

The edk2-platforms project is licensed under the BSD-2-Clause license with
a patent grant, as per commit ae604e4ffe8f ("edk2-platforms: Change
License.txt from 2-Clause BSD to BSD+Patent").

There is a BSD-2-Clause-Patent SPDX license identifier[1] for this case,
therefore refine the edk2-platforms package to use this more specific
identifier.

[1]: https://spdx.org/licenses/BSD-2-Clause-Patent.html



Signed-off-by: Vincent Stehlé <vincent.stehle@arm.com>
Cc: Dick Olsson <hi@senzilla.io>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>