ras|kvm: switch to using generic platform name instead of sgi

To adapt to the product names for the latest SGI platforms, replace the use
of 'sgi' with 'refinfra' for prebuilts.

Change-Id: Id9a1a68606ba510ee639f3378fee9d5e69f83ba7
Signed-off-by: Thomas Abraham <thomas.abraham@arm.com>