cmake: product/sgi575

This change adds CMake build system support for product/sgi575

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.

Signed-off-by: Joel Goddard <joel.goddard@arm.com>
Change-Id: I155b50c3b37b47ac94234c09fb5e14441aeb803b