plat/arm/sgi: Add initial platform support for SGI platforms

System Guidance for Infrastructure are Arm's reference server
platforms. Add mandatory functions and macros required by all
SGI platforms to execute the TF-A tests. The common files
are placed in plat/arm/sgi/common/ folder.

Change-Id: Iaf9f3fa9dfa8d95c883bcab7c1ef00ea228b6b67
Signed-off-by: Chandni Cherukuri <chandni.cherukuri@arm.com>