cmake: product/sgm775

This change adds CMake build support for product/sgm775

Please note that firmware targets do not use libRTX_CM3.a
or RTX_CM3.lib, instead in CMake build, the library
is built using sources.

Change-Id: If5568e88d27689430013b701e0203bc579127ce7
Signed-off-by: Chris Kay <chris.kay@arm.com>
Signed-off-by: Joel Goddard <joel.goddard@arm.com>
Signed-off-by: Girish Pathak <girish.pathak@arm.com>