Adjust libfdt.h to work with swig

There are a few places where libfdt.h cannot be used as is with swig:

- macros like fdt_totalsize() have to be defined as C declarations
- fdt_offset_ptr() and fdt_getprop_namelen() need special treatment due to
    a TODO in the wrapper for fdt_getprop(). However they are not useful to
    Python so can be removed

Add #ifdefs to work around these problem.

Signed-off-by: Simon Glass <sjg@chromium.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>