plat/arm/sgi: add support for SGI-Clark.Ares platform

SGI-Clark.Ares platform is a variant of the SGI-Clark platform. It has
two clusters of four Ares CPUs each. Though very similar to the SGI575
platform, there are subtle differences. HW_CONFIG and TB_FW_CONFIG dts
files have been added.

Change-Id: I740a33cbd1c3b1f1984cb56243b46ad379bab3e6
Signed-off-by: Chandni Cherukuri <chandni.cherukuri@arm.com>