irqchip/meson-gpio: Mark OF related data as maybe unused

The driver can be compile tested with !CONFIG_OF making certain data
unused:

  drivers/irqchip/irq-meson-gpio.c:153:34: error: ‘meson_irq_gpio_matches’ defined but not used [-Werror=unused-const-variable=]

Acked-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20230512164506.212267-1-krzysztof.kozlowski@linaro.org