dm: Provide a function to scan child FDT nodes

At present only root nodes in the device tree are scanned for devices.
But some devices can have children. For example a SPI bus may have
several children for each of its chip selects.

Add a function which scans subnodes and binds devices for each one. This
can be used for the root node scan also, so change it.

A device can call this function in its bind() or probe() methods to bind
its children.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/test/dm/Makefile b/test/dm/Makefile
index c0f2135..5c2415e 100644
--- a/test/dm/Makefile
+++ b/test/dm/Makefile
@@ -5,6 +5,7 @@
 #
 
 obj-$(CONFIG_CMD_DM) += cmd_dm.o
+obj-$(CONFIG_DM_TEST) += bus.o
 obj-$(CONFIG_DM_TEST) += test-driver.o
 obj-$(CONFIG_DM_TEST) += test-fdt.o
 obj-$(CONFIG_DM_TEST) += test-main.o