ARM: dts: vexpress: add missing SPDX GPL-2.0 license identifier

Commit b2441318 ("License cleanup: add SPDX GPL-2.0 license
identifier to files with no license") seem to have missed this
one file.
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>

arch/arm/boot/dts/vexpress-v2m-rs1.dtsi

+// SPDX-License-Identifier: GPL-2.0
 /*
  * ARM Ltd. Versatile Express
  *