libfdt: Implement fdt_get_property_namelen() and fdt_getprop_namelen()

libfdt: Implement fdt_get_property_namelen() and fdt_getprop_namelen()

As well as fdt_subnode_offset(), libfdt includes an
fdt_subnode_offset_namelen() function that takes the subnode name to
look up not as a NUL-terminated string, but as a string with an
explicit length.  This can be useful when the caller has the name as
part of a longer string, such as a full path.

However, we don't have corresponding 'namelen' versions for
fdt_get_property() and fdt_getprop().  There are less obvious use
cases for these variants on property names, but there are
circumstances where they can be useful e.g. looking up property names
which need to be parsed from a longer string buffer such as user input
or a configuration file, or looking up an alias in a path with
IEEE1275 style aliases.

So, since it's very easy to implement such variants, this patch does
so.  The original NUL-terminated variants are, of course, implemented
in terms of the namelen versions.

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