fix(services): drop warning on unimplemented calls

Standard Secure Services, complying to the SMCCC specification, are
discoverable: Any user can do the SMC call, and derive from the return
value (-1) if the service is implemented. Consequently we should not
*warn* if BL31 does not implement a service, as some services (TRNG, for
instance) might never be implemented for devices, as they are lacking
hardware.

Short of dropping the existing warning message altogether, change the
level to VERBOSE, which should prevent it actually being printed in
normal situations.

This removes the pointless TF-A messages on the console when booting
Linux, as modern kernels now call the SOCID and the TRNG service
unconditionally.

Change-Id: I08b0b02e0f46322ebe0b40b3991c3c9b5bed4f97
Signed-off-by: Andre Przywara <andre.przywara@arm.com>

services/std_svc/std_svc_setup.c

@@ -166,7 +166,7 @@ static uintptr_t std_svc_smc_handler(uint32_t smc_fid,
 		SMC_RET2(handle, STD_SVC_VERSION_MAJOR, STD_SVC_VERSION_MINOR);
 
 	default:
-		WARN("Unimplemented Standard Service Call: 0x%x \n", smc_fid);
+		VERBOSE("Unimplemented Standard Service Call: 0x%x \n", smc_fid);
 		SMC_RET1(handle, SMC_UNK);
 	}
 }