libfdt: Add helpers to read #address-cells and #size-cells

This patch makes a small start on libfdt functions which actually help to
parse the contents of device trees, rather than purely manipulating the
tree's structure.

We add simple helpers to read and sanity check the #address-cells and
#size-cells values for a given node.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>