dt-bindings: timer: nuvoton,npcm7xx-timer: Convert to YAML

Let's convert this devicetree binding to YAML, to make it easier to
extend later.

Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220130102704.2892072-1-j.neuschaefer@gmx.net