libfdt: Add fdt_setprop_inplace_namelen_partial

Add a function to modify inplace only a portion of a property..

This is especially useful when the property is an array of values, and you
want to update one of them without changing the DT size.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Acked-by: Simon Glass <sjg@chromium.org>