plat/arm: Remove file arm_board_def.h

This file is shared between FVP and all CSS platforms. While it may be
true that some definitions can be common, it doesn't make sense
conceptually. For example, the stack size depends on the platform and so
does the SRAM size.

After removing them, there are not enough common definitions to justify
having this header, so the other definitions have been moved to the
platform_def.h of FVP, board_css_def.h and arm_def.h.

Change-Id: Ifbf4b017227f9dfefa1a430f67d7d6baae6a4ba1
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>