On DynamIQ CPU FVPs, stats test cases are failing when
hardware-assisted coherency is enabled due to a corrupt
timestamp value. Investigation of the issue indicates that
on these models the timestamp value is stored in cache
instead of memory. This patch flushes the dcache when the
timestamp is stored to make sure it is stored in memory.

Change-Id: I05cd54ba5991a5a96dd07f1e08b5212273201411
Signed-off-by: Zelalem <zelalem.aweke@arm.com>