ARM: vexpress: add support for Versatile Express Cortex-A15-TC2

This adds support for the Cortex-A15-TC2 core tile for the Versatile
Express board by ARM. This is mostly a copy of the A5 support file,
but will be extended later with A15 specific options.

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