Without an explicit cache flush, the next timestamp captured might have
a bogus value.

This can happen if the following operations happen in order,
on a CPU that's being powered down.

1) ENTER PSCI timestamp is captured with caches enabled.

2) The next timestamp (ENTER_HW_LOW_PWR) is captured with caches
   disabled.

3) On a system that uses a write-back cache configuration, the
   cache line that holds the PMF timestamps is evicted.

After step 1), the ENTER_PSCI timestamp is cached and not in main memory.
After step 2), the ENTER_HW_LOW_PWR timestamp is stored in main memory.
Before the CPU power down happens, the hardware evicts the cache line that
contains the PMF timestamps for this service.  As a result, the timestamp
captured in step 2) is overwritten with a bogus value.

Change-Id: Ic1bd816498d1a6d4dc16540208ed3a5efe43f529
Signed-off-by: dp-arm <dimitris.papastamos@arm.com>