dt: eliminate OF_NO_DEEP_PROBE and test for NULL match table

There are no users of OF_NO_DEEP_PROBE, and of_match_node() now
gracefully handles being passed a NULL pointer, so the checks at the
top of of_platform_bus_probe can be dropped.

While at it, consolidate the root node pointer check to be easier to
read and tidy up related comments.

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>