libfdt: Add fdt_path_offset_namelen()

Properties may contain path names which are not NUL-terminated.
For example, the 'stdout-path' property allows the form 'path:options',
where the ':' character terminates the path specifier.

Allow these path names to be used in-place for path descending;
add fdt_path_offset_namelen(), which limits the path name to 'namelen'
characters.

Reimplement fdt_path_offset() as a trivial wrapper.

Signed-off-by: Peter Hurley <peter@hurleysoftware.com>