Boot: Move BL2 shared area definitions to region_defs.h

Move BOOT_TFM_SHARED_DATA_BASE and BOOT_TFM_SHARED_DATA_SIZE macros
definitions from flash_layout.h to region_defs.h in each platform,
for the following reasons:
1. BL2 shared area sits in the RAM area. It is more natural to put
   the definitions in region_defs.h
2. In some topologies and platforms, the base address of BL2 shared
   area is calculated based on other regions in RAM layout during
   compiling. It can be more flexible to put those definitions into
   region_defs.h

Change-Id: Ia846194bdaff1f6e659761042594102a4ad523cb
Signed-off-by: David Hu <david.hu@arm.com>