platform/sgi575: remove the need for a common sgi config file

SGI platforms will have their own platform config file and there
will be no need for a common sgi config file. So remove the common.sgi
file and create a SGI-575 platform specific config file.

Change-Id: Id8febae412a04ca16e040a2f07845e3eb3a76b93
Signed-off-by: Thomas Abraham <thomas.abraham@arm.com>