From now on, platform_def.h must include any header with definitions that
are platform-specific (like arm_def.h) and the included headers mustn't
include back platform_def.h, and shouldn't be used by other files. Only
platform_def.h should be included in other files. This will ensure that all
needed definitions are present, rather than needing to include all the
headers in all the definitions' headers just in case.

This also prevents problems like cyclic dependencies.

Change-Id: I9d3cf4d1de4b956fa035c79545222697acdaf5ca
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>