serial_lpuart: make clock failure less verbose

Some device may enable CONFIG_CLK but not still support this clock in
CC, so better use debug() in place of dev_warn() otherwise a lot of
boards will throw useless dev_warn()s.

Signed-off-by: Giulio Benetti <giulio.benetti@benettiengineering.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

drivers/serial/serial_lpuart.c

@@ -499,7 +499,7 @@ static int lpuart_serial_probe(struct udevice *dev)
 			return ret;
 		}
 	} else {
-		dev_warn(dev, "Failed to get per clk: %d\n",  ret);
+		debug("%s: Failed to get per clk: %d\n", __func__, ret);
 	}
 #endif