plat/arm/sgi: move fdts files to sgi575 board directory

To align the placement of ftds files with that of other Arm platforms,
move the ftds files from plat/arm/css/sgi/ to plat/arm/board/sgi575.

Change-Id: Id7c772eb5cf3d308d4e02a3c8099218e889a0e96
Signed-off-by: Chandni Cherukuri <chandni.cherukuri@arm.com>