FVP: Break circular dependency in platform header files

We used to have the following circular dependency in the FVP platform
header files:

 +-> arm_def.h ---> platform_def.h ---> fvp_def.h --+
 |__________________________________________________|

This patch breaks it by not including 'arm_def.h' from 'fvp_def.h'.

Change-Id: I280d906559e3343dd38764029e77c0ea768b4fec
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>