of: Introduce common early_init_dt_scan

Most architectures scan the all the same items early in the FDT and none
are really architecture specific. Create a common early_init_dt_scan to
unify the early scan of root, memory, and chosen nodes in the flattened
DT.

Signed-off-by: Rob Herring <rob.herring@calxeda.com>
Acked-by: Grant Likely <grant.likely@linaro.org>