hikey960: Fake only 1 core

hikey960 has an unresolved power management bug, so for now we can only
run single core tests. The bug report can be found here:
https://bugs.96boards.org/show_bug.cgi?id=783



Change-Id: I206fafd7677428f81f48d1c7c1d419402149bd37
Co-authored-by: John Tsichritzis <john.tsichritzis@arm.com>
Co-authored-by: Joel Hutton <Joel.Hutton@Arm.com>
Signed-off-by: Joel Hutton <Joel.Hutton@Arm.com>