dm: core: Add a function to bind child devices

dm: core: Add a function to bind child devices

We currently use dm_scan_fdt_node() to bind devices. It is an internal
function and it requires the caller to know whether we are pre- or post-
relocation.

This requirement has become quite common in drivers, so the current function
is not ideal.

Add a new function with fewer arguments, that does not require internal
headers. This can be used directly as a post_bind() method if needed.

Signed-off-by: Simon Glass <sjg@chromium.org>