libfdt: Add fdt_getprop_namelen_w

Add a function to retrieve a writeable property only by the first
characters of its name.

Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>