ARM: virtualization: replace verbose license with SPDX identifier

The original creation of arch/arm/cpu/armv7/{virt-v7.c,nonsec_virt.S}
predates the SPDX conversion, so the original elaborate license
statements sneaked in.
Fix this by replacing them with the proper abbreviation.

Signed-off-by: Andre Przywara <andre.przywara@linaro.org>