of: assume big-endian properties, adding conversions where necessary

of: assume big-endian properties, adding conversions where necessary

Properties in the device tree are specified as big-endian. At present,
the only platforms to support device trees are also big-endian, so we've
been acessing the properties as raw values.

We'd like to add device tree support to little-endian platforms too, so
add endian conversion to the sites where we access property values in
the common of code.

Compiled on powerpc (ppc44x_defconfig & ppc64_defconfig) and arm (fdt
support only for now).

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