dt-bindings: vendor-prefixes: Also allow node names starting with '_'

Generated nodes for overlays begin with '_'. The binding examples are
built as overlays in order to allow unresolved phandles, so we need to
allow the generated node names.

Reviewed-by: Maxime Ripard <maxime.ripard@bootlin.com>
Signed-off-by: Rob Herring <robh@kernel.org>