Add missing utils files to custom OpenAmp CMake

OpenAmp have added a new utils subdirectory with source files that are
needed to build the library so the custom CMake file has been updated
accordingly

Change-Id: I11805d9ac3eb2fc55fadb8f9de92dc47808d75c2
Signed-off-by: Mikael Olsson <mikael.olsson@arm.com>