ppc: qemu: Enable driver model

At present QEMU ppce500 target has not been migrated to driver model
yet. As a start, let's enable driver model and the 'dm' command.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Priyanka Jain <priyanka.jain@nxp.com>

configs/qemu-ppce500_defconfig

@@ -16,6 +16,7 @@ CONFIG_HUSH_PARSER=y
 CONFIG_CMD_REGINFO=y
 CONFIG_CMD_BOOTZ=y
 CONFIG_CMD_GREPENV=y
+CONFIG_CMD_DM=y
 CONFIG_CMD_PCI=y
 CONFIG_CMD_DHCP=y
 CONFIG_CMD_PING=y
@@ -26,6 +27,7 @@ CONFIG_OF_CONTROL=y
 CONFIG_OF_BOARD=y
 CONFIG_ENV_OVERWRITE=y
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
+CONFIG_DM=y
 # CONFIG_MMC is not set
 CONFIG_E1000=y
 CONFIG_SYS_NS16550=y