[DOCS] Add NON_SECURE_SVC_BOOT options in ARM Fast Model bootwrapper

Debugging Xvisor ARM32 on ARM Fast Model can be quite useful in some
secnarios hence, this patch makes our ARM Fast Model bootwrapper
suitable for launching Xvisor ARM32 (without virtualization extension).

To achieve above, we simply add compile time option for ARM Fast Model
bootwrapper which if enabled forces all CPUs to enter Non-secure SVC
mode before jumping into Xvisor.

Signed-off-by: Anup Patel <anup@brainfault.org>