cmake: Ensure artefacts built with presets work with FVP

As CMakePresets are used to run CI tests on FVP,
artefacts built with the `an552-base` CMake preset should always build
for FVP as it uses different peripherals that are selected only when
the CMake variable `VARIANT` is set to `FVP`.
This commit adds the VARIANT CMake variable as a cacheVariable for
the `an552-base` CMake preset. All presets that inherit the
`an552-base` preset will inherit that CMake variable.

Signed-off-by: Hugues Kamba-Mpiana <hugues.kambampiana@arm.com>