of: use __be32 for cell value accessors

Currently, we're using u32 for cell values, and hence assuming
host-endian device trees.

As we'd like to support little-endian platforms, use a __be32 for cell
values, and convert in the cell accessors.

Signed-off-by: Jeremy Kerr <jeremy.kerr@canonical.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>