This patch moves the definition of some macros used only on
ARM platforms from common headers to platform specific headers.
It also forces all ARM standard platforms to have distinct
definitions (even if they are usually the same).
 1. `PLAT_ARM_TZC_BASE` and `PLAT_ARM_NSTIMER_FRAME_ID` have been
     moved from `css_def.h` to `platform_def.h`.
 2. `MHU_BASE` used in CSS platforms is moved from common css_def.h
    to platform specific header `platform_def.h` on Juno and
    renamed as `PLAT_ARM_MHU_BASE`.
 3. To cater for different sizes of BL images, new macros like
    `PLAT_ARM_MAX_BL31_SIZE` have been created for each BL image. All
    ARM platforms need to define them for each image.

Change-Id: I9255448bddfad734b387922aa9e68d2117338c3f