tune-cortexa76ae.inc: Correct TUNE_FEATURES

* as shown by sanity checker:
    Toolchain tunings invalid:
    Tuning 'cortexa76ae' has no defined features, and cannot be used.

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>