ARC: Allow SMP kernel to build/boot on UP-only infrastructure

In light of recent SNAFU with SMP build, allow simple platform to build
as SMP but run UP.

* Remove the dependence on simulation SMP extension to enable quick
  build/test iterations of SMP kernel.

* In absence of platform SMP registration, prevent the NULL smp feature
  name from borkign the system

Signed-off-by: Vineet Gupta <vgupta@synopsys.com>
diff --git a/arch/arc/plat-arcfpga/platform.c b/arch/arc/plat-arcfpga/platform.c
index 1038949..6abc341 100644
--- a/arch/arc/plat-arcfpga/platform.c
+++ b/arch/arc/plat-arcfpga/platform.c
@@ -71,7 +71,7 @@
 	.dt_compat	= ml509_compat,
 	.init_early	= plat_fpga_early_init,
 	.init_machine	= plat_fpga_populate_dev,
-#ifdef CONFIG_SMP
+#ifdef CONFIG_ISS_SMP_EXTN
 	.init_smp	= iss_model_init_smp,
 #endif
 MACHINE_END