arm64: vexpress: Enable ARMv8 RTSM model (SoC) support

This patch adds the necessary Kconfig entries to enable support for the
ARMv8 software model (Versatile Express platform) together with the
defconfig update.

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>