Currently, barebox only parse one memory node which is either the
"/memory" node or the first node with device_type == "memory".
However, the use of multiple memory nodes with device_type = "memory"
property is allowed by the device tree specification and already
correctly parsed by Linux kernel.
In order to fix that, add of_probe_memory function which loop over all
available memory nodes matching device_type == "memory".

Signed-off-by: Clement Leger <cleger@kalray.eu>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>