clk: check parent_name in clk_register to avoid confusing log_error() output

For some gate clocks and fixed clocks without a parent, calling
clk_register will print an useless error message indicating that parent
is missing. Fix that by gaurding log_xxx() with an if-statement.

Signed-off-by: Yang Xiwen <forbidden405@outlook.com>
Suggested-by: Sean Anderson <seanga2@gmail.com>
Reviewed-by: Sean Anderson <seanga2@gmail.com>
Link: https://lore.kernel.org/r/20230807-clk-fix-v2-1-0b688e21fb4e@outlook.com

drivers/clk/clk.c

@@ -18,17 +18,19 @@
 int clk_register(struct clk *clk, const char *drv_name,
 		 const char *name, const char *parent_name)
 {
-	struct udevice *parent;
+	struct udevice *parent = NULL;
 	struct driver *drv;
 	int ret;
 
-	ret = uclass_get_device_by_name(UCLASS_CLK, parent_name, &parent);
-	if (ret) {
-		log_err("%s: failed to get %s device (parent of %s)\n",
-			__func__, parent_name, name);
-	} else {
-		log_debug("%s: name: %s parent: %s [0x%p]\n", __func__, name,
-			  parent->name, parent);
+	if (parent_name) {
+		ret = uclass_get_device_by_name(UCLASS_CLK, parent_name, &parent);
+		if (ret) {
+			log_err("%s: failed to get %s device (parent of %s)\n",
+				__func__, parent_name, name);
+		} else {
+			log_debug("%s: name: %s parent: %s [0x%p]\n", __func__, name,
+				  parent->name, parent);
+		}
 	}
 
 	drv = lists_driver_lookup_name(drv_name);