board: freescale: t4240rdb: implement get_serial_clock

The serial clock is provided by the get_serial_clock() callback on PPC
under DM_SERIAL. Use the same method to compute the clock as for
non-DM_SERIAL use cases.

Signed-off-by: Camelia Groza <camelia.groza@nxp.com>
Signed-off-by: Peng Fan <peng.fan@nxp.com>
diff --git a/board/freescale/t4rdb/t4240rdb.c b/board/freescale/t4rdb/t4240rdb.c
index 0035bd7..ab71776 100644
--- a/board/freescale/t4rdb/t4240rdb.c
+++ b/board/freescale/t4rdb/t4240rdb.c
@@ -1,6 +1,7 @@
 // SPDX-License-Identifier: GPL-2.0+
 /*
  * Copyright 2014 Freescale Semiconductor, Inc.
+ * Copyright 2023 NXP
  */
 
 #include <common.h>
@@ -20,6 +21,7 @@
 #include <asm/fsl_law.h>
 #include <asm/fsl_serdes.h>
 #include <asm/fsl_liodn.h>
+#include <clock_legacy.h>
 #include <fm_eth.h>
 
 #include "t4rdb.h"
@@ -28,6 +30,13 @@
 
 DECLARE_GLOBAL_DATA_PTR;
 
+#if CONFIG_IS_ENABLED(DM_SERIAL)
+int get_serial_clock(void)
+{
+	return get_bus_freq(0) / 2;
+}
+#endif
+
 int checkboard(void)
 {
 	struct cpu_type *cpu = gd->arch.cpu;