lib: arm_ffa: Continue FFA init if MEM_RETRIEVE_REQ is not supported

KVM does not support calling MEM_RETRIEVE_REQ from
a pVM so this commit allows the FFA init hook to
continue if Trusty is running in non-secure.

Bug: None
Change-Id: I5d7c5dbb0631b95001a06f8a562011e5a59a7f5d