of/flattree: Merge earlyinit_dt_scan_root()

Merge common code between PowerPC and Microblaze

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Tested-by: Wolfram Sang <w.sang@pengutronix.de>