of/flattree: merge early_init_dt_scan_chosen()

Merge common code between PowerPC and Microblaze.  This patch
splits the arch-specific stuff out into a new function,
early_init_dt_scan_chosen_arch().

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