Do not enable SVE on pre-v8.2 platforms

Pre-v8.2 platforms such as the Juno platform does not have
the Scalable Vector Extensions implemented and so the build
option ENABLE_SVE is set to zero.

This has a minor performance improvement with no functional
impact.

Change-Id: Ib072735db7a0247406f8b60e325b7e28b1e04ad1
Signed-off-by: David Cunado <david.cunado@arm.com>