dt-bindings: timer: nuvoton,npcm7xx: Add wpcm450-timer

Add a compatible string for WPCM450, which has essentially the same
timer controller.

Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20210320181610.680870-6-j.neuschaefer@gmx.net