ARM platforms: rationalise memory attributes of shared memory

ARM platforms: rationalise memory attributes of shared memory

The shared memory region on ARM platforms contains the mailboxes and,
on Juno, the payload area for communication with the SCP. This shared
memory may be configured as normal memory or device memory at build
time by setting the platform flag 'PLAT_ARM_SHARED_RAM_CACHED' (on
Juno, the value of this flag is defined by 'MHU_PAYLOAD_CACHED').
When set as normal memory, the platform port performs the corresponding
cache maintenance operations. From a functional point of view, this is
the equivalent of setting the shared memory as device memory, so there
is no need to maintain both options.

This patch removes the option to specify the shared memory as normal
memory on ARM platforms. Shared memory is always treated as device
memory. Cache maintenance operations are no longer needed and have
been replaced by data memory barriers to guarantee that payload and
MHU are accessed in the right order.

Change-Id: I7f958621d6a536dd4f0fa8768385eedc4295e79f