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>

board/freescale/qemu-ppce500/qemu-ppce500.c

@@ -374,3 +374,9 @@ void *board_fdt_blob_setup(void)
 {
 	return get_fdt_virt();
 }
+
+/* See CONFIG_SYS_NS16550_CLK in arch/powerpc/include/asm/config.h */
+int get_serial_clock(void)
+{
+	return get_bus_freq(0);
+}