dm: firmware: Automatically bind child devices

To support scenarios where a firmware device node has subnodes that
have their own drivers automatically scan the DT and bind those when
the firmware device gets bound.

Reviewed-by: Tom Rini <trini@konsulko.com>
Signed-off-by: Andreas Dannenberg <dannenberg@ti.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>