dt-bindings: media: amphion,vpu: correct node name

The node name should use hyphen(-), not underscore(_).

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>