hikey960: Increase max xlat tables

It is cleaner to use the same number of tables regardless of whether
OP-TEE is used or not. This doesn't result in a significant memory
usage increase.

Change-Id: I38bcaa3f2f197c0a352153117592749f189a265c
Signed-off-by: Joel Hutton <Joel.Hutton@Arm.com>