arm64: Slightly improve the warning on CPU0 enable-method

Commit e8765b26 (arm64: read enable-method for CPU0) introduced
checks for the enable method on CPU0 (to be later used with CPU
suspend). However, if the kernel is compiled for UP and a DT file is
used with a method like 'spin-table', Linux complains about 'invalid
enable method'. This patch turns it into an 'unsupported enable method'
warning.
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/kernel/cpu_ops.c

@@ -68,7 +68,7 @@ int __init cpu_read_ops(struct device_node *dn, int cpu)
 
 	cpu_ops[cpu] = cpu_get_ops(enable_method);
 	if (!cpu_ops[cpu]) {
-		pr_err("%s: invalid enable-method property: %s\n",
+		pr_warn("%s: unsupported enable-method property: %s\n",
 			dn->full_name, enable_method);
 		return -EOPNOTSUPP;
 	}