serial: ns16550: Handle -ENOENT when requesting clock

When calling clk_get_by_index(), fall back to the legacy method of
getting the clock if -ENOENT is returned.

Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Tested-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Thierry Reding <treding@nvidia.com>

drivers/serial/ns16550.c

@@ -408,7 +408,7 @@ int ns16550_serial_ofdata_to_platdata(struct udevice *dev)
 		err = clk_get_rate(&clk);
 		if (!IS_ERR_VALUE(err))
 			plat->clock = err;
-	} else if (err != -ENODEV && err != -ENOSYS) {
+	} else if (err != -ENOENT && err != -ENODEV && err != -ENOSYS) {
 		debug("ns16550 failed to get clock\n");
 		return err;
 	}