ARM: mach-virt: add SMP support using PSCI

This patch adds support for SMP to mach-virt using the PSCI
infrastructure.

Acked-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Nicolas Pitre <nico@linaro.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>