Include board_arm_def.h through the platform's header

The board_arm_def.h header file needs to be included via the platform
definition header. Not doing so, results in a redefinition error of
PLAT_ARM_MAX_BL31_SIZE macro, if defined in the platform definition
file.

Change-Id: I1d178f6e8a6a41461e7fbcab9f6813a2faa2d82b
Signed-off-by: Sughosh Ganu <sughosh.ganu@arm.com>