boards: arm: add armfvp support for ns variants

What is the change?
  - Non secure build variants can now be used with ARMFVPs.

Why is this change needed?
  - Running non secure variant of a build doesn't work with FVPs
    previous to version 11.27. The newer version of FVP added the
    support to run the tfm_merged.hex files.

Signed-off-by: Sudan Landge <sudan.landge@arm.com>