of/flattree: Add non-boottime device tree functions

In preparation for providing run-time handling of device trees, factor
out some of the basic functions so that they take an arbitrary blob,
rather than relying on the single boot-time tree.

V2:
- functions have of_fdt_* names
- removed find_flat_dt_string
- blob argument is first

Signed-off-by: Stephen Neuendorffer <stephen.neuendorffer@xilinx.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>