of/flattree: Add of_flat_dt_match() helper function

This patch adds of_flat_dt_match() which tests a node for
compatibility with a list of values and converts the relevant powerpc
platform code to use it.  This approach simplifies the board support
code a bit.

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