bootm: Do not cross 1MiB sections for the devicetree
ARM Linux only maps a single 1MiB section for the devicetree. This has
a 1Mib alignment, so we are not allowed to cross such a boundary. Align
the devicetree to the next power of two so that this never happens.
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>