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ARM Trusted Firmware keeps certain data structures in a memory region
with non-cacheable attributes (termed as "coherent memory") to keep data
coherent with observers that are cache-coherent, and those not. These
data structures pertain to power management and mutual exclusion. Using
coherent memory also costs at least an additional page to map memory
with special memory attributes.

On systems with hardware-assisted coherency, all CPUs that participate
in power management and mutual exclusion are cache-coherent, obviating
the need for special memory attributes for such data structures.
Instead, they can be placed in normal memory, along with rest of data.

On systems with hardware-assisted coherency, where build option
HW_ASSISTED_COHERENCY will be set, also having USE_COHERENT_MEMORY
enabled only wastes a page of memory without any
benefit. Therefore, with HW_ASSISTED_COHERENCY set to 1, require that
USE_COHERENT_MEMORY is explicitly set to 0.

Change-Id: I5101657ae6b1a46278069f23e2d88ee5cbd98efa
Signed-off-by: Jeenu Viswambharan <jeenu.viswambharan@arm.com>