powerpc: Add serial driver for driver model

This uses the ns16550 driver but sets up the clock at run-time. It does
not seem to be available in the device tree.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/serial/Makefile b/drivers/serial/Makefile
index 4cc00cd..63b0cbf 100644
--- a/drivers/serial/Makefile
+++ b/drivers/serial/Makefile
@@ -8,6 +8,7 @@
 ifdef CONFIG_DM_SERIAL
 obj-y += serial-uclass.o
 obj-$(CONFIG_PL01X_SERIAL) += serial_pl01x.o
+obj-$(CONFIG_PPC) += serial_ppc.o
 else
 obj-y += serial.o
 obj-$(CONFIG_PL010_SERIAL) += serial_pl01x.o