It is not always desired to get the devicetree from image data and load
it to a SDRAM region at the same time. Sometimes it's enough to just
load it to an allocated address (in case the user has no constraints
where the devicetree should be placed.

This patch splits bootm_load_devicetree into bootm_get_devicetree which
returns a pointer to the allocated devicetree and bootm_load_devicetree
which loads the devicetree to a specified region.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>