armhf, xsm: Drop XSM tests on 32-bit ARM

armhf, xsm: Drop XSM tests on 32-bit ARM

We are at the limit of ARM capacity, which has not kept pace with X86
expansion.  New hardware is on the way but it is not here yet.

Dropping the XSM tests drops one test, but also one build, which saves
considerable effort.  Builds are currently the bottleneck, because we
avoid building on the unreliable Arndale boards.

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
Acked-by: Julien Grall <julien.grall@arm.com>