HW_ASSISTED_COHERENCY implies something stronger than just hardware
coherent interconnect, specifically a DynamIQ capable ARM core.

For K3, lets use WARMBOOT_ENABLE_DCACHE_EARLY to enable caches early
and then let the caches get shut off on powerdown, to prevent data
corruption we also need to USE_COHERENT_MEM so that any accesses to
shared memory after this point is only to memory that is set as
non-cached for all cores.

Change-Id: Ib9337f012df0e0388237942607c501b6f3e2a949
Signed-off-by: Andrew F. Davis <afd@ti.com>