build-test-tftf: allow this script to be usable on sgiclarka platform

SGI-Clark.Ares uses a TF-A firmware to boot the application cores. So
TF-A tests can be used on this platform to validate the port of TF-A on
this platform. So allow the usability of this script on SGI-Clark.Ares
platform.

Change-Id: I9fb77167d9a446f65db6bdabb970fac13d4f0e1f
Signed-off-by: Chandni Cherukuri <chandni.cherukuri@arm.com>