plat/arm/sgi: reorganize platform macros

In preparation of adding support for upcoming SGI platforms, macros
common to all the SGI platforms are moved into sgi_base_platform_def.h
file. Macros that are specific to sgi575 platform remain in the
platform_def.h file. In addition to this, the platform_def.h file is
moved to sgi575 board directory. Also the ENT_CPU_SOURCES has been
renamed to SGI_CPU_SOURCES and moved from sgi-common.mk to board
specific makefile platform.mk

Change-Id: Iccdd9f070f4feea232b9fbf4fdcc0ef2e8eccbf2
Signed-off-by: Chandni Cherukuri <chandni.cherukuri@arm.com>