dt-bindings: interrupt-controller: ti,omap4-wugen-mpu: Add file extension

Add TXT file extension so the format of binding will be explicit (we
have also bindins in YAML).

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20250107131111.246969-1-krzysztof.kozlowski@linaro.org
Signed-off-by: Rob Herring (Arm) <robh@kernel.org>

Author: krzk