ARM: VExpress: enable ARMv7 virt support for VExpress A15

To enable hypervisors utilizing the ARMv7 virtualization extension
on the Versatile Express board with the A15 core tile, we add the
required configuration variable.
Also we define the board specific smp_set_cpu_boot_addr() function to
set the start address for secondary cores in the VExpress specific
manner.
There is no need to provide a custom smp_waitloop() function here.

This also serves as an example for what to do when adding support for
new boards.

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