plat/arm/sgi: remove unused code

On SGI platforms, the interconnect is setup by the SCP and so the
existing unused interconnect setup in sgi575 platform code can be
removed. As a result of this, sgi_plat_config.c and sgi_bl1_setup.c
files can be removed as these files are now empty or can be
substainated by the existing weak functions.

Change-Id: I3c883e4d1959d890bf2213a9be01f02551ea3a45
Signed-off-by: Chandni Cherukuri <chandni.cherukuri@arm.com>