FVP_Base_AEMv8A platform: Fix cache maintenance operations

FVP_Base_AEMv8A platform: Fix cache maintenance operations

This patch fixes FVP_Base_AEMv8A model hang issue with
ARMv8.4+ with cache modelling enabled configuration.
Incorrect L1 cache flush operation to PoU, using CLIDR_EL1
LoUIS field, which is required by the architecture to be
zero for ARMv8.4-A with ARMv8.4-S2FWB feature is replaced
with L1 to L2 and L2 to L3 (if L3 is present) cache flushes.
FVP_Base_AEMv8A model can be configured with L3 enabled by
setting `cluster0.l3cache-size` and `cluster1.l3cache-size`
to non-zero values, and presence of L3 is checked in
`aem_generic_core_pwr_dwn` function by reading
CLIDR_EL1.Ctype3 field value.

Change-Id: If3de3d4eb5ed409e5b4ccdbc2fe6d5a01894a9af
Signed-off-by: Alexei Fedorov <Alexei.Fedorov@arm.com>