arch/arm64: Fix compilation without CONFIG_SMP

Compilers aren't happy with a declaration of boot_cpu() after defining
boot_cpu to 1.

Change-Id: I22d0db61646f3e226e5996fa94223ffbb6b760e5
Signed-off-by: Patrick Georgi <pgeorgi@chromium.org>
Reviewed-on: http://review.coreboot.org/10696
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>

src/arch/arm64/cpu.c

@@ -220,7 +220,9 @@ void arch_cpu_wait_for_action(void)
 	}
 }
 
+#if IS_ENABLED(CONFIG_SMP)
 int boot_cpu(void)
 {
 	return cpu_is_bsp();
 }
+#endif