dm: Add functions to access a device's children

Devices can have childen that can be addressed by a simple index, the
sequence number or a device tree offset. Add functions to access a child
in each of these ways.

The index is typically used as a fallback when the sequence number is not
available. For example we may use a serial UART with sequence number 0 as
the console, but if no UART has sequence number 0, then we can fall back
to just using the first UART (index 0).

The device tree offset function is useful for buses, where they want to
locate one of their children. The device tree can be scanned to find the
offset of each child, and that offset can then find the device.

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