ppc: qemu: Switch over to use DM serial

The QEMU ppce500 target integrates 2 NS16550 serial ports. Switch
over to use the DM version of the driver by:

- drop unnecessary ad-hoc config macros
- add get_serial_clock() in the board codes

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Priyanka Jain <priyanka.jain@nxp.com>
diff --git a/configs/qemu-ppce500_defconfig b/configs/qemu-ppce500_defconfig
index f2a8b83..c528a68 100644
--- a/configs/qemu-ppce500_defconfig
+++ b/configs/qemu-ppce500_defconfig
@@ -30,6 +30,7 @@
 CONFIG_DM=y
 # CONFIG_MMC is not set
 CONFIG_E1000=y
+CONFIG_DM_SERIAL=y
 CONFIG_SYS_NS16550=y
 CONFIG_ADDR_MAP=y
 CONFIG_PANIC_HANG=y