cmake: product/sgm776

This adds CMake build system support for product/sgm776

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: I8726c51054452fc800882067d521d51bb253bc05