dt-bindings: timer: renesas: mtu2: Convert to json-schema

Convert the Renesas Multi-Function Timer Pulse Unit 2 (MTU2) Device Tree
binding documentation to json-schema.

Add missing properties.
Update the example to match reality.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Rob Herring <robh@kernel.org>