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>
3 files changed