fdt_support: split fdt_getprop_u32_default

We already have a nice helper to give us a property cell value with default
fall back from a path. Split that into two helpers - one for the old path
based lookup and one to give us a value based on a node offset.

Signed-off-by: Alexander Graf <agraf@suse.de>
Acked-by: Scott Wood <scottwood@freescale.com>
Reviewed-by: York Sun <yorksun@freescale.com>