sgi/sgi_common_util: add helper functions to check for supported platforms

The sgi test scripts have replicated code that validates in the input
parameters with the list of platforms supported. This duplicated code
is now moved into a sgi common code as appropriate functions.

Change-Id: I0cc4ea8b6186adc1e3919f861f009caf09d36df1
Signed-off-by: Thomas Abraham <thomas.abraham@arm.com>